BY CNN Wire Staff
Top U.S. officials closely monitored the apparent collapse of Libyan leader Moammar Gadhafi's government 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.
President Barack Obama - currently on vacation in Martha's Vineyard, Massachusetts - issued a written statement late Sunday calling on Gadhafi to "relinquish power once and for all."
"The momentum against the Gadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant," Obama said. "The Gadhafi regime is showing signs of collapsing. The people of Libya are showing that the universal pursuit of dignity and freedom is far stronger than the iron fist of a dictator."
"The surest way for the bloodshed to end is simple: Moammar Gadhafi and his regime need to recognize that their rule has come to an end," he said. "A season of conflict must lead to one of peace."
White House officials said Obama has been receiving frequent updates on the crisis in part from John Brennan, his counterterrorism 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, White House Deputy Press Secretary Josh Earnest said. White House Chief of Staff William Daley, National Security Adviser Tom Donilon, Secretary of Defense Leon Panetta, and Chairman of the Joint Chiefs Adm. Mike Mullen were among those participating in the call, according to Earnest.
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, Alan Silverleib, and Barbara Starr contributed to this report