The United States is preparing to accede to Pakistani demands that it vacate a remote air base in Pakistan used for drone flights, but the move is not expected to have a significant impact on operations against militants, US government sources say.

Washington is treading lightly not to aggravate an already fragile relationship that was bruised further by a Nato attack on a Pakistani military outpost last weekend that killed 24 Pakistani soldiers near the Afghanistan border.

Pakistan demanded that the United States leave the Shamsi Air Base within 15 days and blocked ground supply routes through Pakistan to US forces in Afghanistan.

Three sources, who declined to be identified because of the issue's sensitivity, said US planning is under way to leave the base, a remote facility in Balochistan that has been a point of contention.

The cross-border incident escalated tensions between the two countries and the US military is conducting an investigation to find out exactly what happened on the ground.

The moves by the Pakistanis to block ground supply routes and the air base were not expected to significantly hinder US operations.

One US government source said the United States has spent months preparing for a possible eviction from the Pakistan base by building up other drone launching and staging capability.

Earlier this year, after the US raid that killed al Qaeda leader Osama bin Laden, some Pakistani officials demanded that Washington vacate the Shamsi facility.

At the time, however, US officials said that American personnel would remain at the base and would continue to conduct drone flights in pursuit of militants.

But in one concession, the United States stopped conducting lethal drone operations from that base and limited operations to surveillance flights.

US officials believe that this time Pakistan appears much more resolute about carrying out the eviction threat.

Vacating the air base was seen more as an inconvenience rather than a critical blow to drone operations which the United States also conducts from Afghanistan and possibly elsewhere.

BLOCKED SUPPLY ROUTEThe United States also has to deal with the blocking of the ground supply route through Pakistan to Afghanistan.

US Congressman C.A. “Dutch” Ruppersberger, the senior Democrat on the House of Representatives intelligence committee, said that route accounts for less than half the supplies for international forces in Afghanistan and the military has contingency plans.

“We have a large distribution network to make sure that coalition forces are well-stocked,” he told Reuters. “It's not going to affect our ability to follow through and execute our mission.”

Yet alternate supply routes such as the northern distribution network are not a perfect substitute and there are concerns that the cost of keeping soldiers fed, armed and fueled without use of Pakistani roads would be excessive.

Ruppersberger, who visited Pakistan to meet with officials after US forces killed bin Laden, said the relationship was poor at that point.

“We were starting to improve in the last month or so and then all of a sudden this unfortunate incident occurred, and now we're right back to where we were again,” he said.

“It is to the advantage of both countries to work together,” Ruppersberger said. “In the end that will come. It's about relationships, it's about trust, and unfortunately that hasn't been there for a while.”

Ruppersberger would not comment on the Shamsi departure.

STILL INVESTIGATINGUS officials said there is still considerable confusion about details of the latest border incident.

Wary of further damaging an already delicate situation, US officials were reluctant to speculate about what happened before getting the results of military investigations.

“The focus of the administration at this point is on trying to find ways to show Pakistan that we're serious about investigating the incident and forging a cooperative relationship in the future,” a US official said on condition of anonymity.

“No one at this point has the complete narrative on what happened,” Pentagon spokesman said.