Published: Nov 27, 2011 10:51 PM EST

WASHINGTON (AP) - The U.S. is planning its own investigation into NATO's deadly airstrikes in Pakistan, while two senior lawmakers called for tough diplomacy after Islamabad turned away supply convoys into Afghanistan and demanded that the U.S. vacate a drone base.

Gen. James Mattis, head of U.S. Central Command, which oversees American military operations in the region, was expected by Monday to name an investigating officer to examine the incident, according to a defense official who was not authorized to discuss the matter publicly and spoke on condition of anonymity.

A key question to be examined by the U.S. is who approved the airstrikes and why.

The attack could become the deadliest friendly fire incident against Pakistani troops since the war began a decade ago. It also raises serious questions about the extent of cooperation between supposed close allies in fighting terrorism.

"There's a lot of diplomacy that has to occur and it has to be tough diplomacy in the sense that they need to understand that our support for them financially is dependent upon their cooperation with us," said Sen. Jon Kyl of Arizona, the Senate's No. 2 Republican.

Afghan officials say their soldiers called for help on Saturday after being fired upon from the direction of Pakistani border posts. Pakistani authorities claim the airstrikes were unprovoked.

NATO has said it is conducting an investigation of the incident. The alliance has not commented on Pakistani claims that the attacks killed 24 soldiers, but it has not questioned them.

Alliance officials previously have complained that insurgents fire from across the poorly defined frontier, often from positions close to Pakistani soldiers, who have been accused of tolerating or supporting them.

The incident threatens to send U.S.-Pakistani relations to an all-time low.

U.S. officials were already reeling in the wake of the raid in May on Osama bin Laden's hideout in a Pakistani garrison town. The Pakistan government was outraged it hadn't been told about the operation beforehand, and U.S. secrecy surrounding the operation underscored a deep mistrust between the two allies.

Frustration is particularly acute among members of Congress, who amid an economic recession are being asked to support billions in military and civilian aid for Pakistan.

Sen. Dick Durbin of Illinois, the second-ranking Democrat, said Pakistan's latest move to punish coalition forces for the airstrikes is further evidence that the U.S. must end its military involvement in the region and bring troops home.

"As difficult as it is to fight our way thru this diplomatic morass between the incompetence and maybe corruption of Afghanistan and the complicity in parts of Pakistan, our soldiers are caught right in the middle of this at a time they are trying to bring peace to the region," Durbin said.

While calling for tougher diplomacy with Pakistan, Kyl said he would stop short of cutting off U.S. aid entirely to Pakistan. He said that severing ties in the past has only led to an increased influence of Islamic extremists among Pakistan's military ranks.

"It's very important to maintain the relationship for the long haul," he said, without offering more specifics on how that might be done.

Durbin suggested the U.S. back out from the region from a military standpoint.

"We've got to leave it to Afghan forces," he said.

Kyl and Durbin spoke on "Fox News Sunday."

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