BRUSSELS, June 12: US Defence Secretary Robert Gates invited Pakistan on Thursday to join an investigation into an air strike that killed 11 Pakistani soldiers, saying he regretted it had created problems with a key ally.

Gates said he had not personally spoken to Pakistanis, but the US military had. Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, has been in contact with his counterpart in Pakistan.

“We agree we need to investigate this incident,” Gates said. “We’ve invited the Afghans and the Pakistanis to be a part of the investigation. Pakistan is an incredibly important partner for us in this war on terror.

“Personally, I regret we’ve had something that has created a problem. We’ve had an incident that has created a problem between us and the government of

Pakistan,” he said.

President George W. Bush’s national security adviser, on the other hand, said US officials “have not been able to corroborate” claims by Pakistani officials that a US skirmish with militants along the Afghan-Pakistani border killed 11 Pakistani troops.

“Should it be true, obviously we would be very saddened by that loss,” national security adviser Stephen Hadley told reporters travelling with Bush in Europe.

Reflecting how the incident has put the already fragile US-Pakistan relationship on shaky ground, Hadley noted that Pakistan has been an important ally in the war on terror, and added that “we hope” that will continue.

“One of the problems is that it is still not exactly clear what happened,” he said. “The reports, quite frankly, even from sources within the United States, are conflicting.”

Hadley said that the account the US believes is true is this: There was an operation on the Afghan side of the border by “anti-coalition forces” that threatened coalition personnel. The forces then went “back into Pakistan” and the coalition fighters “tracked and struck those forces”.

“That’s what we believe happened,” Hadley said.

Pakistan has lodged a strong diplomatic protest over what it says was an air strike on Tuesday on one of its border posts in a frontier region that killed 11 of its troops.

However, the US military has released excerpts of a video shot by a surveillance drone circling above the mountainous battle zone. The grainy images show about a half-dozen men firing from a ridge at coalition troops off-camera in a valley below.

A voiceover said the ridge was in Afghanistan and that the coalition forces were on a reconnaissance mission.

The video shows the “anti-Afghan militants” moving to a position identified as inside Pakistan and the impact of a bomb which the voiceover says killed two of them. The survivors then fled into a ravine, where three more bombs were dropped, nearly three hours after the clash began. —Agencies