(CNN) -- The extremist group ISIS "probably" won't be defeated under the current Obama administration, Deputy National Security Adviser Tony Blinken told CNN's Wolf Blitzer on Wednesday.
"This, as the President has said, is going to have to be a sustained effort," Blinken said. "It's going to take time, and it will probably go beyond even this administration to get to the point of defeat."
After the beheading of journalist Steven Sotloff by ISIS militants, Obama faced a chorus of bipartisan calls from Congress to act more aggressively and come up with a concrete strategy to combat ISIS, beyond the ongoing targeted airstrikes in Iraq.
Nudges from Congress
Florida Democratic Sen. Bill Nelson announced on Tuesday that he plans to introduce legislation making it clear the President has the authority for airstrikes against ISIS in Syria.
Rep. Frank Wolf, a Republican from Virginia, said he plans to offer an even broader measure that gives the President the power to use military force against international terrorists.
Last week, the Senate's top Republican told CNN that he would support congressional authorization for a new, broadened strategy to go after ISIS.
"In all likelihood, it would make sense for (President Obama) to get our support," said Sen. Mitch McConnell of Kentucky. "It's pretty clear ISIS is a serious threat. They have the potential to hit us here at home."
Still, House Speaker John Boehner is putting the onus on the President to present a strategy before Congress acts.
"The President is the commander in chief ... and it's his responsibility as the chief executive to outline a plan that will protect American interests, protect American lives -- both at home and abroad," Boehner said in a radio interview this week.
"Until the President is willing to lay out a plan, the Congress has very few options ahead of it," Boehner said.
House Majority Leader Kevin McCarthy echoed the Speaker's concerns.
"First and foremost ... the President has to have a strategy.," McCarthy said Wednesday on the Hugh Hewitt radio show. "I mean what is our foreign policy? I don't know what it is and if we don't know what it is and our allies don't know, and our enemies don't know so they are pushing the envelope."
Clarifying his position
Speaking Wednesday, Obama addressed his much criticized statement last week that he has no strategy on ISIS in Syria. He said he was referring to a military strategy in Syria that "might" require congressional approval.
"Our objective is clear. That is to degrade and destroy (ISIS) so it's no longer a threat," he said. "We can accomplish that. It's going to take some time, it's going to take some effort."
The President said the world needs a regional strategy to defeat the group.
"We've been putting together a strategy that was designed to do a number of things... What we have to make sure is we have a regional strategy in place," he said.
CNN's Deirdre Walsh and Dana Bash contributed to this report