WASHINGTON: 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.”

In the wake of the strikes, Pakistan decided not to take part in the Bonn Conference on the future of Afghanistan that opens Monday, a decision which, together with the Taliban's boycott, has cast the event's usefulness into doubt.

Pakistan has also shut down Nato's vital supply line into Afghanistan and ordered American personnel to leave Shamsi air base.

The base is widely understood to have been a hub for the covert CIA drone war on Taliban and Al-Qaeda commanders in Pakistan's troubled border areas with Afghanistan.