U.S. personnel have taken steps to evacuate a remote airfield in Pakistan that had been used for staging classified drone flights directed against militants, U.S. and Pakistani sources said on Monday.

Following a NATO airstrike last month during which 24 Pakistani troops were killed accidentally, Pakistan ordered U.S. personnel to vacate the airfield at Shamsi in its Baluchistan region by December 11.

After receiving this ultimatum - which initially was transmitted to the Obama administration in the form of a press release - the United States began preparing for a possible move of American personnel out of the facility.

On Monday, a Pakistani military official, who requested anonymity because he is not authorized to speak to the media, told Reuters: There is some activity happening at the base because of the deadline given to the Americans. They are moving some equipment and vacating personnel.

Two other sources close to the Pakistani and American governments confirmed that U.S. personnel had begun marshalling personnel and equipment to be moved out of the base in the event the Pakistanis do not relent on their demand.

We're not going to comment specifically on Shamsi but we will comply with the Pakistani requests, Pentagon press secretary George Little said on Monday.

The Pakistani airbase had been used by U.S. forces, including the CIA, to stage elements of a clandestine U.S. counter-terrorism operation to attack suspected encampments of militants associated with al Qaeda, the Taliban and Pakistan's home-grown Haqqani network, using unmanned drone aircraft armed with missiles.

President Barack Obama stepped up the drone campaign after he took office. U.S. officials say it has produced major successes in decimating the central leadership of al Qaeda and putting associated militant groups on the defensive.

Pakistani authorities started threatening U.S. personnel with eviction from the Shamsi base in the wake of the raid last May in which U.S. commandos killed Osama bin Laden at his hide-out near Islamabad without notifying Pakistani officials in advance.

In an attempt to mollify the Pakistanis, U.S. authorities began limiting drone flights from the base to nonlethal surveillance flights. But in the wake of the latest deadly NATO air strike, Pakistani authorities renewed and stepped up pressure on the U.S. to vacate the base entirely.

Obama on Sunday called Pakistan's President Asif Ali Zardari to offer condolences over a NATO airstrike that killed the 24 Pakistani troops and provoked a crisis in relations between the two countries.

(Reporting by Mark Hosenball in Washington and Gul Yusufzai and Asim Tanveer in Pakistan; editing by Anthony Boadle)