The U.S. decision to vacate a remote base used for staging drone flights directed at militants will have little impact on the American air campaign, a senior Pakistani military source said on Monday, but the move is a big political win for Pakistan.

U.S. personnel completed their departure from Shamsi Airfield, 320 km (200 miles) southwest of Quetta in the wild Baluchistan province. The operation began on about Dec 5, after a NATO cross-border strike killed 24 Pakistani soldiers.

That strike enraged Pakistan, allowing it push through a longstanding demand that the Americans leave Shamsi.

But a Pakistani military source said their departure would have little impact on the drone campaign, which is hugely unpopular in Pakistan.

The Shamsi base was more of a maintenance and refuelling base for the drones, while most of the operations are conducted from across the border, he said.

Technically speaking, yes, the vacation of this base should have an impact on the drone operations, but it remains to be seen as to how big an impact this would be.

The source said the United States had at least two other bases in Afghanistan it could use.

The last U.S. personnel left the base at about 12:15 p.m., according to a senior security official who requested anonymity because he was not authorised to speak to the media.

Twenty-one cargo flights in the last 10 days helped with the evacuation. Pakistani security forces now control the base.

Pakistanis saw the U.S. departure as a significant victory.

It's a big move politically, and does show that we are asserting ourselves and won't tolerate any such acts any more, the senior military source said.

It also shows that the military as well as the government are on the same note on this issue, and so I will call it a significant move to get the base vacated.

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 and the Taliban, 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 after 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 non-lethal surveillance flights. But following the latest deadly NATO air strike, Pakistani authorities renewed and stepped up pressure on the United States to vacate the base entirely.

Pakistan's Prime Minister Yusuf Raza Gilani, in an interview with the BBC, said Pakistan could take further retaliatory action, including possibly closing its airspace to the United States.

(Writing by Chris Allbritton; Additional reporting by Gul Yusufzai in QUETTA; Editing by Ron Popeski)