The United States stressed the "importance" of its ties with Pakistan after up to 26 soldiers were killed in cross-border Nato air strikes on Saturday, plunging already frosty relations into crisis.
In a joint statement, US secretary of defense Leon Panetta and secretary of state Hillary Clinton offered their "deepest condolences" and said they backed "Nato's intention to investigate immediately."
"Secretaries Clinton and Panetta have been closely monitoring reports of the cross-border incident in Pakistan today," the statement said.
"Both offer their deepest condolences for the loss of life and support fully Nato's intention to investigate immediately."
Islamabad has ordered a review of all arrangements with the United States and Nato, including diplomatic, political, military and intelligence activities, following the deadly cross-border strikes.
The US-led Nato force in Afghanistan admitted it was "highly likely" that the force's aircraft caused the pre-dawn deaths, inflaming US-Pakistani relations still reeling from the May killing of Osama bin Laden.
Pakistan also told the United States to vacate a remote air base reportedly used as a hub for covert CIA drone strikes on areas bordering Afghanistan, though US officials have said no US military personnel are based there.
Clinton, General John Allen, the US commander in Afghanistan, and the chairman of the US Joint Chiefs of Staff, General Martin Dempsey, called their Pakistani counterparts to discuss the situation.
"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 US statement said.
"All these leaders pledged to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time."