Pakistani troops are accused of ambushing American and Afghan soldiers four years ago, an attack that was apparently covered up to protect a fragile alliance.
The ambush, which had been previously blamed on insurgents, left an American soldier dead, according to a leaked report.
Analysts said the timing of the leak was significant, coming just as the US heaps pressure Pakistan over its failure to tackle militant groups, and further evidence of a breakdown in relations between the warring allies.
The fresh account comes from Afghan and American officers who were present during the assault, according to a New York Times report.
They had crossed into Pakistan at Teri Mangal on May 14, 2007, to discuss a border dispute that had already provoked bloody clashes.
The five-hour meeting at a schoolhouse had ended and the team was preparing to leave when Pakistani forces unleashed a calculated assault, according to the witnesses.
They returned fire before escaping in a Black Hawk helicopter. As well as one fatality, three Americans and an Afghan were wounded.
Colonel Sher Ahmed Kuchai, the Afghan border guard commander, said he saw at least two Pakistanis firing from the open windows of the classrooms.
“It did not last more than 20 seconds, but this was a moment of life and death,” he said.
Initially Pakistan said insurgents were to blame before revising the story to suggest it was the work of a single, rogue member of the Frontier Corp.
“The official line covered over the details in the interests of keeping the relationship with Pakistan intact,” said a former United Nations official who served in Afghanistan and was briefed on the events.
The new account has come to light as US and Pakistan relations reach new depths over Islamabad’s links to the al Qaeda-linked Haqqani network.
Last week, Admiral Mike Mullen, Washington’s most senior military man, said the Haqqanis were acting as a “veritable arm” of the Inter-Services Intelligence directorate and accused Pakistan of “exporting violence”.
His words were part of a rare, public salvo from senior Obama administration officials, suggesting their patience was wearing thin.
Hasan Askari Rizvi, a Pakistani political analyst, said the latest allegations were part of a propaganda war.
“This is clearly an attempt to put more pressure on Pakistan and demonstrate that Pakistan has been acting in this nasty manner for at least four years,” he said. “There may be more stories like this to come – from Pakistan as well.”