Washington (CNN) -- With a flurry of comments and activity, U.S. officials sought Tuesday to lay the groundwork for a military strike in Syria -- though some in Washington are pushing back.
The moves by top members of President Barack Obama's administration come less than a week after rebels claim more than 1,300 people were killed in a Syrian government, most of them dying from use of chemical weapons.
The White House offered legal justification for a strike, with spokesman Jay Carney telling reporters the large-scale use of chemical weapons in Syria presented a national security threat to the United States that required a response.
Carney said Obama had yet to make a final decision on how to respond to what U.S. officials characterize as the worst chemical weapons attack since former Iraqi strongman Saddam Hussein launched a poison gas attack that killed thousands of Kurds in 1988.
The president continues to review options, Carney said, adding "nothing has been decided." Carney assured reporters some sort of response will come.
"Allowing the use of chemical weapons on a significant scale to take place without a response would present a significant challenge to or threat to the United States' national security," he said.
Vice President Joe Biden made clear the administration's view of who was to blame, telling the American Legion that "there is no doubt who is responsible for the heinous use of chemical weapons -- the Syrian regime."
On the same day Obama talked with Canadian Prime Minister Stephen Harper and UK Prime MInister David Cameron, Secretary of Defense Chuck Hagel and Secretary of State John Kerry consulted allies and indicated potentially imminent action by a coalition likely to include key NATO partners and regional powers.
The United States has already moved warships armed with cruise missiles into the region. Hagel told the BBC on Tuesday that forces were ready to carry out a strike if ordered. A senior Defense Department official told CNN that any strike could be completed "within several days."
"We are ready to go, like that," Hagel told the BBC, adding that "the options are there, the United States Department of Defense is ready to carry out those options."
White House: No decision yet
Options available to Obama range from ordering limited missile strikes to continued diplomatic efforts labeled by critics as a "do-nothing" approach.
The White House has ruled out sending ground troops to Syria or implementing a no-fly zone to blunt al-Assad's aerial superiority over rebels fighting to oust his regime.
On Monday, Carney said that the first step toward a military response in Syria would be the public release of a U.S. intelligence report on the August 21 event near Damascus that reportedly killed and wounded thousands.
A U.S. official who was not authorized to speak on the record told CNN that release of the intelligence report was planned for Tuesday, but Carney later said it would come out some time this week.
Another official told CNN the intelligence report would include forensic evidence and intercepted communications among Syrian military commanders.
Yet there's been some debate, within the administration, over what information should be released. The CIA and other intelligence agencies, for instance, have argued there was no need, and perhaps harm, in divulging details, two U.S. officials told CNN's Evan Perez.
One of the officials noted that the U.S. conclusion that the Syrian government was responsible, as expressed by Biden and Kerry, made releasing underlying intelligence superfluous.
Carney said Tuesday there was "no doubt" in the administration that chemical weapons were used by the al-Assad government, telling reporters that "we see no evidence of any alternative scenario."
For almost two years, Obama has avoided direct military involvement in Syria's civil war, only escalating aid to rebel fighters in June after suspected smaller-scale chemical weapons attacks by Syrian government forces.
However, last week's attack obliterated the "red line" Obama set just over a year ago against the use of Syria's chemical weapons stocks.
Legislator: Congress can't 'be pushed aside'
Yet some in Congress say that before Obama orders any strike, they should go through them first.
A group of 31 Republican and six Democratic lawmakers on Tuesday sent a letter to the president urging him "to consult and receive authorization" before authorizing any such military action.
"Any U.S. military action could bring serious consequences or further escalation," said Rep. Ed Royce, a California Republican who chairs the House Foreign Affairs Committee and was not among the letter's initial signatories. "The president should be making the case to the American public, and his administration should come to Congress to explain their plans.
"The consequences are too great for Congress to be pushed aside."
Rep. Peter King, a New York Republican and influential member of the House Homeland Security Committee, said the U.S. response to the Syria situation will influence another regional proliferation issue -- Iran's potential development of nuclear weapons.
Obama "time and again has basically said that Iran will not be allowed to have nuclear weapons, that's the red line," King told CNN on Monday, adding that failure to enforce a similar threat against Syria on chemical weapons would undermine the president.
"This is as much a warning to Iran as I see it, as it is action against Syria," King said.
Even some in Obama's own party are cautioning against action that's not suitably inclusive. Sen. Jack Reed of Rhode Island told CNN on Monday the United States should only act in concert with an international coalition from at least NATO allies and Arab League members.
"Without their participation, it looks as if this is just a Western-vs.-Islamic struggle. It's not," said Reed, a member of the Senate Armed Services Committee. "This is to vindicate a basic rule of international law that these weapons will not be used, not by Iran, not by any power."
Reed said the most realistic option would be cruise missiles launched from U.S. Navy ships at sea, noting that "we can have precision weapons that could be fired and keep our aircraft out of Syrian airspace and away from their anti-aircraft systems."
"The most effective targets would have command-and-control, because you could send a signal to the Syrian regime that if they don't agree to international standards, if they don't make it clear and make it obvious that they're not going to use these weapons, and that we can inflict additional damage on their command-and-control," he added.
Earlier, a senior administration official said that assuming Obama decides to go ahead with a military response, any action could come as early as mid-week.
Factors weighing into the timing of any action include a desire to get it done before the president leaves for Russia next week for a summit with G8 allies, and before the administration has to make a decision on whether to suspend aid to Egypt because of the ongoing political turmoil there, the official explained.
American officials are consulting with allies to ensure they are supportive of any U.S. action, which the senior administration official said would be very limited in scope and a direct reaction to the use of chemical weapons. Representatives of three allied governments involved in those top-level consultations said the goal is to reach a consensus as soon as possible.
"No one is talking about a long process," one European diplomat told CNN.
Russia, China opposition make U.N. action unlikely
While U.N. Secretary-General Ban Ki-moon said Monday that the use of chemical weapons was a crime against humanity and must be punished, certain opposition by Syrian ally Russia and possibly China undermined the possibility that the Security Council would support a military mission.
Instead, a limited coalition of NATO partners such as Germany, France and Britain -- all of which have called for action against Syria -- and some Arab League members appeared more likely to provide political backing for Obama to order U.S. missile strikes.
Marie Harf, a State Department spokeswoman, said Kerry spoke Monday with his British, Jordanian, Qatari and Saudi counterparts, as well as the secretary-general of the Arab League.
The diplomatic efforts appeared to be working, with an Arab League spokesman condemning the al-Assad regime Tuesday for the chemical attack.
In another move, the United States postponed its involvement in talks scheduled for this week in Geneva on seeking a political solution to the Syrian civil war. Russia expressed disappointment at the U.S. decision and warned against any Western military strike on Syria.
Last month, Joint Chiefs Chairman Gen. Martin Dempsey provided Congress with a list of declassified U.S. military options for Syria that emphasized the high costs and risks of what he said would amount to "an act of war" at a time of deep budget cuts.
Dempsey's letter, dated July 19, listed U.S. assets in the region including Patriot missile defense batteries in Turkey and Jordan, as well as F-16 jet fighters positioned to defend Jordan from possible cross-border trouble. In addition, the Pentagon has sent four warships armed with cruise missiles to the region.
According to U.S. officials, updated options offered to the president in recent days included:
• Cruise missiles fired from one of four Navy destroyers deployed in the Mediterranean Sea. The missiles would be used to strike "command and control" facilities such as command bunkers, or the Syrian regime's means of delivering chemical weapons: artillery batteries and launchers. There is no indication that the missiles would strike actual chemical weapons stockpiles.
• Military jets firings weapons from outside Syrian airspace. This option carries additional risks and is considered less likely.
To Aaron David Miller, a vice president at the Woodrow Wilson International Center, the situation is forcing Obama to shift from being an "avoider-in-chief" regarding military involvement in Syria.
"It's almost inevitable that the president will authorize some form of military action," Miller told National Public Radio in an interview broadcast Monday.
He said he expected a significant response that amounts to "a warning that lays down this time a red line that the president intends to enforce, not one that turns pink."
"It cannot simply be a couple of cruise missiles into a storage shed somewhere," Miller said, adding that the goal was to deter al-Assad rather than topple him or radically shift the balance in Syria at this time. "The president's not on the verge of becoming the cavalry to rescue the country."
CNN's Chris Lawrence and Jill Dougherty reported on this story, which was written by Tom Cohen. CNN's Frederik Pleitgen, Elise Labott, Greg Botelho, Hamdi Alkhshali and Ben Brumfield contributed to this report.