President Obama repeated calls for Libyan leader Moammar Gadhafi to step down, and recognized the opposition leadership after rebel forces captured parts of the capital city Tripoli.

In a statement released Sunday evening, the president said: "[T]he momentum against the Qadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant."

Obama again called on Gadhafi to relinquish power. “The surest way for the bloodshed to end is simple: Moammar Qadhafi and his regime need to recognize that their rule has come to an end. Qadhafi needs to acknowledge the reality that he no longer controls Libya. He needs to relinquish power once and for all,” the statement read.

Obama also announced that the United States had recognized the opposition Transitional National Council as "the legitimate governing authority," and urged them “to continue to demonstrate the leadership that is necessary to steer the country through a transition.”

The president is on a 10-day vacation at Martha's Vineyard, but White House officials said the president is receiving briefings from his national security advisers and monitoring the developing situation.

In a statement, White House deputy press secretary Josh Earnest said "the President has received regular updates from John Brennan about the situation in Libya."

"Shortly after 9pm EDT tonight, the President conducted a conference call with senior members of his national security team who briefed him on the dramatic developments there. The President asked that he continue to be updated as necessary and is scheduled to be briefed on this topic again tomorrow morning," Brennan added.

Chief of Staff William Daley, National Security Adviser Tom Donilon, Defense Secretary Leon Panetta and Chairman of the Joint Chiefs of Staff Michael Mullen were participants on the call.

The president received word of the rebels' advance into Tripoli at a briefing Sunday morning from Deputy National Security Adviser John Brennan.

“We believe that Gadhafi’s days are numbered, and that the Libyan people deserve a just, democratic and peaceful future,” Earnest said Sunday afternoon.

Earlier Sunday, a State Department spokesman also repeated calls for Gadhafi to stand down. "If Gadhafi cared about the welfare of the Libyan people, he would step down now," the spokesman said as reported by The Associated Press.

According to reports, Libyan rebels claimed to have seized areas of the Libyan capital Tripoli from forces loyal to Gadhafi. The Libyan opposition also claims to have captured two of the dictator's sons.

The White House said the administration was in close contact with Libya’s opposition.

British Prime Minister David Cameron said Monday that Gadhafi’s regime was “falling apart and in full retreat.” He called on him to “stop fighting, without conditions.”

Cameron confirmed that NATO forces “have no confirmation of Gadhafi’s whereabouts, but at least of his Gadhafi’s sons have been detained.”

In a press conference Sunday, a Libyan government official criticized NATO for the loss of life. "We hold Mr. Obama, Mr. Cameron and Mr. [French President Nicolas] Sarkozy morally responsible for every single unnecessary death that takes place in this country," spokesman Moussa Ibrahim said.

The government vowed the capital would be defended. “We have thousands of professional soldiers and thousands of volunteers protecting the city," Ibrahim added.

Speaking on CBS’s "Face the Nation" on Sunday, Sen. John McCainJohn Sidney McCainLawmakers worry about rise of fake video technology Democrats put Dreamers and their party in danger by playing hardball Trump set a good defense budget, but here is how to make it better MORE (R-Ariz.) predicted it was a "matter of hours, if not days" until Gadhafi’s downfall.

McCain cautioned that Western nations would need to stay heavily involved, saying that the future of the country would be “up to us and the Europeans.” “We are going to have to help out a lot,” he said.

As events unfolded Sunday, McCain tweeted "Qaddafi on his way out, Bashar al Assad is next."

Sen. Joe Lieberman (I-Conn.) tweeted: "The fall of #Tripoli, happening now, marks a historic turning pt. in the Arab Spring and a victory for the cause of freedom around the world."

Rep. Keith Ellison (D-Minn.) praised the opposition forces. "Great wishes of hope for people of Libya. You won the civil war; all the best on winning the peace! Bless Libya's patriots," he tweeted.

—This report was first published Aug. 21 at 3:00 p.m. and has been updated.