Situation has risen over a NATO air attack on Saturday that left 24 Pakistani soldiers dead, Pakistan has shut off supply routes to US-led forces in Afghanistan.

Even if the Pakistanis make good on their threat over Shamsi, US officials and analysts say the move would be largely symbolic as Washington could fly Predator and Reaper drones out of air fields in neighbouring Afghanistan.

"Shamsi is a nice thing to have, but it's not critical to drone operations. They can be carried out from bases in Afghanistan," said Bruce Riedel, a former CIA officer and fellow at the Brookings Institution think tank.

The remote Shamsi air base in the country's southwest is particularly useful for flights hampered by poor weather conditions, he said.

A senior US official said the facility was not a make-or-break link for the robotic planes that have proved an effective weapon against Al-Qaeda and Taliban.

So far there was no sign that Islamabad would bar the US aircraft from flying over Pakistan, and its announcement on Shamsi appeared designed to placate a domestic audience in Pakistan, officials said.

The Shamsi base reflects the contradictions in the uneasy partnership between the two countries, with Islamabad reluctant to publicly acknowledge its tacit cooperation with US counter-terror efforts, which many Pakistanis see as a violation of their country's sovereignty.

"You have to have jet fuel delivered to Shamsi," Riedel said. "The Pakistani public has the impression of a base that operates extraterritorially but in reality it operates because the Pakistani army helps it to operate."

Although President Barack Obama's administration was working on a response to a number of demands from Pakistan, there were no plans to pull back on the drone raids, which intelligence officials have credited with weakening the Al-Qaeda network.

"Pakistan remains a critical counter-terrorism partner, and we do not anticipate significant changes in that relationship," another US official said.

A more serious problem for the United States and NATO allies is Pakistan's decision to close its border to convoys ferrying fuel and supplies to coalition troops in landlocked Afghanistan.

Nearly half of all cargo bound for NATO-led forces runs through Pakistan. Roughly 140,000 foreign troops, including about 97,000 Americans, rely on supplies from outside Afghanistan for the ten-year-old war effort.

Pakistan has shut off the border over previous incidents, partly to allay popular outrage, and US officials said they expected the latest closure would be temporary.

The Pentagon said top government officials and commanders are working with the Pakistanis "on a way ahead" following the air strikes and the White House underscored the importance of the relationship with Islamabad.

Despite the deep distrust between the United States and Pakistan, neither country can afford a complete rupture in relations, officials said.

"By permanently cutting off supplies to NATO forces, Pakistan would not just be taking on the United States but NATO and the United Nations," Riedel said.

Copyright AFP (Agence France-Presse), 2011