The Obama administration on Saturday pledged a full investigation into a NATO attack that allegedly killed 24 Pakistani troops.
Secretary of State Hillary Rodham Clinton and Defense Secretary Leon Panetta in a joint statement offered their "deepest condolences" for the loss of life in the cross-border incident in Pakistan. Clinton and Panetta also said they "support fully NATO's intention to investigate immediately."
Secretary Clinton, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, and Gen. John Allen, commander of the NATO-led coalition forces in Afghanistan, each called their Pakistani counterparts as well, the statement said.
U.S. Ambassador to Pakistan Cameron Munter also met with Pakistani government officials in Islamabad.
"In their contacts, these US diplomatic and military leaders each stressed -- in addition to their sympathies and a commitment to review the circumstances of the incident -- the importance of the US-Pakistani partnership, which serves the mutual interests of our people," the statement said.
"All these leaders pledged to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time," the statement concluded.
The incident was a major blow to American efforts to rebuild an already tattered alliance vital to winding down the 10-year-old Afghan war. It was the latest in a series of setbacks to the alliance, often caused by border incidents.
Islamabad called the bloodshed in one of its tribal areas a "grave infringement" of the country's sovereignty.
If confirmed, it would be the deadliest friendly fire incident by NATO against Pakistani troops since the Afghan war began a decade ago.
A NATO spokesman said it was likely that coalition airstrikes caused Pakistani casualties, but an investigation was being conducted to determine the details.
The relationship between Pakistan and the U.S. has severely deteriorated over the last year, especially following the covert American raid that killed Osama bin Laden in a Pakistani garrison town in May. Islamabad was outraged it wasn't told about the operation beforehand.
The border issue is a major source of tension between Islamabad and Washington, which is committed to withdrawing its combat troops from Afghanistan by the end of 2014.
Much of the violence in Afghanistan is carried out by insurgents who are based just across the border in Pakistan. Coalition forces are not allowed to cross the frontier to attack the militants. However, the militants sometimes fire artillery and rockets across the line, reportedly from locations close to Pakistani army posts.