Continued from page 1

At the White House, spokesman Jay Carney said President Obama considers the Pakistani deaths a tragedy, and he said the administration is determined to investigate.

The Pentagon released a four-page memo from Gen. Mattis directing Gen. Clark to determine what happened, which units were involved, which ones did or did not cross the border, how the operation was coordinated, and what caused the deaths and injuries.

Gen. Mattis also asked Gen. Clark to develop recommendations about how border operations could be improved, and said the final report should be submitted by Dec. 23.

The details of the airstrike emerged as aftershocks were reverberating across the U.S. military and diplomatic landscape Monday, threatening communications and supply lines for the Afghan war and the success of an upcoming international conference.

While U.S. officials expressed regret and sympathy over the cross-border incident, they are not acknowledging blame, amid conflicting reports about who fired first.

The airstrike was politically explosive as well as deadly, coming as U.S. officials were working to repair relations with the Pakistanis after a series of major setbacks, including the U.S. commando raid into Pakistan that killed Osama bin Laden in May.

In recent weeks, military leaders had begun expressing some optimism that U.S.-Pakistan military cooperation along the border was beginning to improve. U.S. Army Maj. Gen. Daniel Allyn told Pentagon reporters just last Tuesday that incidents of firing from Pakistan territory had tapered off somewhat in recent weeks.

Speaking to reporters Monday, Pentagon press secretary George Little stressed the need for a strong military relationship with Pakistan.

“The Pakistani government knows our position on that, and that is we do regret the loss of life in this incident, and we are investigating it,” Mr. Little said.

The military fallout began almost immediately.

Pakistan has blocked vital supply routes for U.S.-led troops in Afghanistan and demanded Washington vacate a base used by American drones. Pakistan ordered CIA employees to mothball their drone operation at Pakistan’s Shamsi Air Base within two weeks, a senior Pakistani official said. The officials spoke on condition of anonymity to discuss intelligence matters.

On the diplomatic front, Pakistan said Tuesday it will boycott an international conference on Afghanistan next week to protest the incident.

The decision to boycott the Bonn, Germany, conference was made during a Pakistani Cabinet meeting in the city of Lahore, said three officials who attended the meeting. They spoke on condition of anonymity because they were not authorized to talk to the media ahead of an official announcement.

The State Department also issued a new warning for U.S. citizens in Pakistan. It said some U.S. government personnel working in Pakistan were being recalled to Islamabad and warned Americans to be on guard for possible retaliation. U.S. citizens in Pakistan are being told to travel in pairs, avoid crowds and demonstrations, and keep a low profile.

Associated Press writers Lolita C. Baldor, Bradley Klapper and Pauline Jelinek contributed to this report.