President Obama Briefed on Libya While Vacationing in Martha's Vineyard

U.S. leaders monitored the progress of Libyan rebel forces on Sunday as they pressed ahead toward Tripoli and pounded on the doorstep of leader Muammar al-Qaddafi's home base.

The White House said President Barack Obama was briefed Sunday morning on the latest developments by counterterrorism adviser John Brennan and heard reports from U.S. teams on the ground in the rebel stronghold of Benghazi.

Rebel forces were said to be fewer than 20 miles outside Tripoli, where they captured a key military base that protects the capital. An Associated Press reporter with the rebels saw them take over the base of the Khamis Brigade, 16 miles west of Tripoli.

White House spokesman Josh Earnest said the U.S. will remain in close contact with its allies and Libyan rebel leaders. Brennan is with the president as he vacations on Martha's Vineyard, Massachusetts.

U.S. officials have been saying they believe Qaddafi's days in power are numbered, and they are poised to assist the opposition as the country moves toward democracy. But it wasn't clear Sunday how close such a transition might be, as Qaddafi and his supporters have vowed to continue the fight.

Looking ahead to a possible rebel victory in the 6-month-old civil war, U.S. Sen. John McCain said it will be very difficult to build a united democratic government there, due to the tribal rivalries.

"We've seen the difficulties with other countries who made this transition, but we will be rid of a guy who has the blood of Americans on his hands. We will be rid of a guy who has practiced the worst kind of brutalities. And now it's going to be up to us and the Europeans," said the Arizona Republican senator, speaking on CBS television's "Face the Nation."

McCain said he thinks the opposition forces can ultimately succeed in setting up a new government, aided by revenues from oil. He said he believes it is a "matter of hours, if not days" before Qaddafi falls.