President Barack Obama on Sunday expressed condolences to Pakistan’s President Asif Ali Zardari for the deaths last month of 24 Pakistani troops, saying the NATO airstrikes that killed them were not a “deliberate attack.”

A White House statement said Obama placed a call early Sunday to Zardari expressing his regrets over the “tragic loss” and promising a “full investigation” into the incident, which has plunged the two uneasy allies into a diplomatic crisis.

Obama “made clear that this regrettable incident was not a deliberate attack on Pakistan and reiterated the United States’ strong commitment to a full investigation,” the statement said.

Islamabad has so far refused to take part in a US investigation into the November 26 air strikes on the Afghan border.

But the White House said Obama and Zardari nonetheless “reaffirmed their commitment to the US-Pakistan bilateral relationship, which is critical to the security of both nations, and they agreed to stay in close touch.”