Washington (CNN) -- President Barack Obama said Monday that the regime of Moammar Gadhafi in Libya "is coming to an end" and he pledged U.S. support for a democratic Libya.
In a statement to reporters from his vacation in Martha's Vineyard, Massachusetts, Obama said the situation remained fluid on the ground in Libya, but that it was clear "Gadhafi's rule is over."
"The situation in Libya has reached a tipping point," Obama said, adding that celebrations as rebels of the National Transitional Council attempt to take control of Tripoli show "that the pursuit of human dignity is far stronger than any dictator."
Obama praised the "unprecedented" international coalition that responded to what he called Gadhafi's brutal aggression against his own people.
At the same time, Obama warned of the danger of reprisals and cited the importance of conciliation, calling for an "inclusive transition that will lead to a democratic Libya." The United States would be a friend and a partner of such a government, he said.
As events in Libya have shown, "fear can give way to hope, and the power of people striving for freedom can bring about a brighter day," the president said.
U.S. officials told CNN that initial signs from the attempted rebel takeover of Tripoli were encouraging, with no reports of any widespread looting or people taking justice into their own hands for retribution.
"I'm encouraged by these reports that they've set up checkpoints to promote public safety and around public buildings," said Assistant Secretary of State for Near Eastern Affairs Jeffrey Feltman. "Tripoli does not look like Baghdad looked after the fall of Saddam Hussein and I think that's encouraging."
Separately, a senior U.S. official, who spoke on background because of the diplomatic sensitivity of the issue, told CNN that Libya's NTC has a series of plans that are "quite detailed" about running public services in an interim period" as well as how to make sure that the government is properly inclusive.
In a telephone interview from Cairo, Feltman said Libyan officials with whom the United States had previous contact were still trying to reach out to the Obama administration as recently as Saturday night.
However, Feltman said, the Libyan officials were taking a "defiant" approach and refused to negotiate Gadhafi's departure.
"I think they were looking for a way to find a lifeline, buy time, to prevent what was then becoming inevitable, which was the uprising in Tripoli," said Feltman, who leads State Department efforts on Libya and who was in the rebel stronghold of Benghazi over the weekend.
Top U.S. officials closely monitored the Libya situation Monday, expressing concern that forces loyal to the longtime strongman might launch a last-ditch offensive against that country's civilian population.
"There is reason to believe (Gadhafi) remains in Libya" and may still be able "to issue orders" to his troops through a limited communications network, a U.S. official told CNN.
NATO authorities expressed a similar concern.
"If there is a last-ditch effort we want to protect civilians," a senior NATO official said, speaking on the condition of anonymity because of sensitive intelligence matters.
The official said NATO is watching closely for any sign of a massing of Gadhafi's forces, or of weapons such as rockets or artillery being moved. Striking such targets in the heavily populated areas of Tripoli could be a difficult problem because rebel forces, civilians and loyalists are mixed in among the entire population he said.
Obama issued a written statement late Sunday calling on Gadhafi to "relinquish power once and for all," and he repeated that call on Monday.
White House officials said Obama has been receiving frequent updates on the crisis in part from John Brennan, his counter-terrorism adviser. Brennan has been receiving intelligence from a team in the rebel stronghold of Benghazi, the White House noted.
Obama held a high-level call with senior members of his national security team Sunday evening, according to White House Deputy Press Secretary Josh Earnest, and another call with the group took place Monday before Obama issued his statement.
White House Chief of Staff William Daley, National Security Adviser Tom Donilon, Defense Secretary Leon Panetta and Chairman of the Joint Chiefs Adm. Mike Mullen were among those participating in the call, according to Earnest.
On Sunday night, Obama told reporters he would issue a statement when there was more clarity on the situation on the ground.
"I think overnight and this morning we have gotten some greater clarity and confirmation about circumstances on the ground on the situation in Libya and Tripoli in particular," Earnest said Monday.
Obama first called on Gadhafi to step aside in February, but Gadhafi held on until this weekend despite the imposition of new sanctions and growing pressure from a NATO-led air campaign.
American forces played a critical role in establishing and supporting NATO's campaign, despite widespread U.S. public opposition to any American involvement. Only 35% of Americans supported U.S. military action in Libya in a July 18-20 CNN/ORC International Poll. A solid majority -- 60% -- opposed American military intervention in the North African nation.
A sharply divided Congress failed to agree on any resolutions explicitly endorsing or opposing U.S. involvement. Two top GOP senators issued a statement Sunday praising the collapse of Gadhafi's regime, but criticizing Obama for failing to act in a more aggressive manner.
"The end of the Gadhafi regime in Libya is a victory for the Libyan people and for the broader cause of freedom in the Middle East and throughout the world. This achievement was made possible first and foremost by the struggle and sacrifice of countless Libyans, whose courage and perseverance we applaud," said Sens. John McCain of Arizona and Lindsey Graham of South Carolina.
"Americans can be proud of the role our country has played in helping to defeat Gadhafi, but we regret that this success was so long in coming due to the failure of the United States to employ the full weight of our airpower," they said.
Gadhafi spokesman Musa Ibrahim, meanwhile, blamed Obama, NATO and the West for the conflict, and appealed for a cease-fire.
"Every drop of Libyan blood shed by these rebels is the responsibility of the Western world, especially NATO's countries," Ibrahim said. "We hold Obama, (British Prime Minister David) Cameron and (French President Nicolas) Sarkozy morally responsible for every single unnecessary death that takes place in this country."
CNN's Dan Lothian, Jill Dougherty, Alan Silverleib, Tom Cohen and Barbara Starr contributed to this report.