U.S. officials were in frequent contact Monday with Libyan rebels as they claimed control of most of the capital city of Tripoli. The whereabouts of longtime Libyan leader Moammar Gadhafi's were still unknown, but White House and the Pentagon officials said they believe he's still in the country.

The White House said Obama would make remarks about the developments in Libya at 2 p.m. EDT Monday.

Following the rebel's lightning advance on Tripoli, President Barack Obama urged Gadhafi to recognize his time is over in Libya. And Assistant Secretary of State Department Jeffrey Feltman said it was "only a matter of time" before the besieged ruler is history.

Still, Feltman acknowledged in an interview from Cairo Monday morning that U.S. officials do not know where Gadhafi is. And White House spokesman Josh Earnest, echoing the Pentagon, told reporters "there's no evidence to think he has left."

Earnest said Obama was updated early Monday by John Brennan, his deputy national security adviser, on the rebel seizure of much of Tripoli. Obama is also hosting a secure conference call with his national security team from his rented farm complex on the island off Cape Cod.

Asked whether the rebel advances were a vindication of Obama's strategy to let NATO take the lead in Libya, Earnest said he would not "assess winners and losers," but said Obama's approach "has yielded a lot of favorable results here."

He said the administration had no intention of changing its policy of keeping U.S. troops out of Libya.

Amid celebrations among rebels and sympathizers on the streets of Tripoli, Feltman said he thought it was "very clear that the rebels are winning."

"The rebels are taking over the city. They are clearly taking over the institutions," Feltman said in an interview on ABC's "Good Morning America." He also said U.S. officials have been told the rebels have seized control of state television.

Asked whether he believes the al Qaida terrorist network will gain new footing during a power vacuum in Libya, Feltman said the first step in any post-Gadhafi setting is to "prevent some kind of cycle where people act out their own retributions," as happened when Saddam Hussein fell in Iraq.

"A lot of that sectarian mix that existed in Saddam Hussein's Iraq doesn't exisit here in Libya," Feltman said. He also said that "the overwhelming vision that we are hearing" from people across Libya is that "they want a Libya that is moderate, that is secular."

Secretary of State Hillary Rodham Clinton telephoned the leader of the Libyan Transitional National Council on Monday, and also spoke to leaders of several nations who are part of an international diplomatic effort known as the Libya Contact Group. That body could meet as soon as next week in Europe.

A State spokesman said Monday that no decision has yet been made on whether to send U.S. military and diplomatic weapons experts to Libya to help prevent the Gadhafi regime's massive arsenal of antiaircraft missiles from slipping into the hands of terror groups.

Military officials estimate the regime amassed as many as 30,000 Man-Portable Air Defense Systems, or MANPADS, before the fighting began. Most are believed to be early-model Russian missiles and launchers.

The U.S. has already sent an interagency team to the region to confer with Libya's neighbors and is providing $3 million to two international weapons abatement teams to locate and dispose of the weapons.

Some missiles have been dismantled, but officials in Algeria and several other nations have raised alarms that other plundered weapons have reached al-Qaeda's North African branch.

In Washington Monday, At the Pentagon, spokesman Bryan Whitman said officials were carefully assessing developments.

"Clearly, there's a fluid situation," Whitman said. "We are monitoring the situation closely." The Pentagon has provided well over 60 percent to 70 percent of the intelligence flights in support of NATO operations involving Libya. The U.S. led airstrikes before turning the mission over to NATO forces.

A vacationing Obama said in a statement from Martha's Vineyard, Mass., Sunday night that Gadhafi should relinquish power to stop the violence and bloodshed of six months of civil war aimed at toppling his autocratic regime.

"The future of Libya is now in the hands of the Libyan people," Obama said. The U.S. has said that it would work closely with the rebels.

After a day of dramatic developments, Obama said the situation in Libya had reached a "tipping point" and control of the capital was "slipping from the grasp of a tyrant."

"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," Obama said. "Gadhafi needs to acknowledge the reality that he no longer controls Libya. He needs to relinquish power once and for all." Obama issued the statement after conducting a conference call with members of his national security team, who had provided him with updates throughout the day.

Libyan rebels who raced into Tripoli on Sunday met little resistance as Gadhafi's defenders melted away and his 42-year authoritarian rule quickly crumbled. Euphoric fighters celebrated with residents of the capital in Green Square, the symbolic heart of the fading regime. Gadhafi's whereabouts were unknown, though state TV broadcast his bitter pleas for Libyans to defend his regime.

Opposition fighters captured his son and one-time heir apparent, Seif al-Islam, who along with his father faces charges of crimes against humanity at the International Criminal Court in the Netherlands. Another son was in contact with rebels about surrendering, the opposition said.

"Tonight, the momentum against the Gadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant," Obama said in the statement. "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 United States has joined other countries in recognizing the the Transitional National Council as the legitimate government in Libya.

Obama called on the rebels "at this pivotal and historic time" to demonstrate the leadership needed to steer the country through a transition by respecting the rights of the Libyan people, avoiding civilian casualties, protecting state institutions and pursuing a transition to democracy that is "just and inclusive" for all of the country's people.

Obama said the U.S. would remain in close contact with the TNC and work with its allies and partners around the world to protect the Libyan people and support a peaceful shift to democracy.

Secretary of State Hillary Rodham Clinton and Defense Secretary Leon Panetta were also kept updated throughout the day, officials said.


Associated Press writers Mark S. Smith and Erica Werner in Vineyard Haven, Mass., and Matthew Lee, Lolita C. Baldor, Pauline Jelinek, Stephen Braun and Julie Pace in Washington contributed to this report.