ISLAMABAD - Pakistan kept a vital border crossing closed to U.S. and NATO supply trucks for a third day Saturday, a sign that Islamabad's desire to avoid a domestic backlash over a NATO incursion that killed three Pakistani troops is - for now - outweighing its desire to stay on good terms with America.

Two U.S. missile strikes that killed 16 people in a northwest Pakistani tribal region, meanwhile, showed that America has no intention of sidelining a tactic it considers highly successful, even if it could add to tensions.

The closing of the Torkham border crossing to NATO trucks has exposed the struggles and contradictions at the heart of the U.S.-Pakistan alliance against Islamist militancy.

Both sides need one another: The U.S. gives billions in military and other aid to Pakistan, and the U.S. and NATO use Pakistani roads to transport the majority of their non-lethal supplies to troops in Afghanistan.

But while the U.S.-led coalition is busy tackling every insurgent group they can along the Pakistani-Afghan border before America's scheduled withdrawal from Afghanistan starting in mid-2011, Pakistan has only gone after certain groups sheltering on its side - the ones it deems most dangerous to its government, not to Westerners in Afghanistan.

A recent surge in the CIA's drone-fired missile strikes in Pakistan along with NATO operations along the frontier suggest Western forces are testing how far they can push Pakistan.

Tensions came to a head after helicopters from the military alliance were alleged to have crossed the border multiple times last weekend in pursuit of insurgents, killing dozens of militants with airstrikes. Pakistan protested to NATO and threatened to stop aiding the coalition's convoys into Afghanistan. On Thursday, it made good on its threat to cut the supply line after two NATO helicopters killed three Pakistani paramilitary soldiers who fired warning shots at them.

On Saturday, some 150 trucks piled up near the border crossing at Torkham, waiting for the post to reopen. The truck drivers said they were worried, as militants are known to attack the supply line on a fairly regular basis.

"We are suffering and feeling fear," driver Hayat Zaman said.

Analysts said they expected the border to be closed for two or three more days at least. Pakistan would look like it was backing down if it reopened the border too quickly, but at the same time, it wouldn't risk its partnership with the United States by keeping the crossing closed for too long, they said.

"The whole thing is so political, they cannot quickly reopen," said Ayesha Siddiqa, a military analyst.

Siddiqa said the Pakistani army may also view this as a chance to nudge the U.S. more toward its thinking on how to deal with the range of militant groups that operate in its border regions - namely, that it's better for Islamabad to focus for now on fighting militants attacking Pakistanis as opposed to those attacking Western troops in Afghanistan.