WASHINGTON — U.S. airstrikes into Pakistan that may have accidentally killed allied fighters have upset the already fragile relations between Washington and Islamabad over how to stem violence in the lawless border region.

Defense Department press secretary Geoff Morrell defended the bombing Wednesday and said it was too early to know whether the strike killed 11 Pakistani paramilitary forces, as alleged by the angry Pakistani Army.

"Every indication we have is that this was a legitimate strike against forces that had attacked members of the coalition," he told a Pentagon press conference.

Other U.S. officials said earlier Wednesday that three aircraft launched about a dozen bombs into Pakistan after militants attacked coalition forces in a wooded area near a checkpoint. Conflicting reports about the Tuesday clash were being sorted out by the U.S. military.

U.S. Ambassador Anne Patterson was summoned to the Foreign Ministry, where Pakistan's government also lodged a diplomatic protest.

"The United States regrets that actions ... on the night of June 10 resulted in the reported casualties among Pakistani forces who are our partners in the fight against terrorism," a U.S. Embassy statement said. It expressed condolences to the families of the dead.

Rich Barton, a Pakistan expert at the Center for Strategic and International Studies, said the incident comes at a bad time, when the new Pakistani government is already overwhelmed trying to find its way.

"The bad news with this kind of an incident is that it really distracts from the more important transition that's going on in Pakistan and it could really be exploited as an organizing tool to get people back to thinking the United States is the root cause" of problems in their country, Barton said.

"It could easily be used as a provocation for some of the groups that are most anti-American and are outside the government as well," he said.

The incident has inflamed anti-U.S. sentiment in Pakistan, where a new government is trying to reach out to tribal leaders in the border region to negotiate a peace deal. U.S. officials have expressed skepticism about the plan, and there have been repeated questions about Pakistan's commitment and ability to battle terrorists known to be hiding in the mountainous terrain.

As recently as Monday, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, repeated U.S. fears that if left unchecked, the ungoverned border region will likely spawn the next attack on U.S. soil. That area is also where some believe Osama bin Laden is hiding.

Mullen, who has visited Pakistan three times since February, has talked at length with Gen. Ashfaq Kayani, who replaced stalwart U.S. ally President Pervez Musharraf as Pakistan Army chief last year.

The government, Mullen said, recognizes the challenge and is looking for the best way to handle it.

"I am learning as I go that these tribal areas are extraordinarily complex. There's no simple answer," said Mullen, adding that the U.S. wants peace agreements that can be enforced so that no insurgents cross the border.

Details of the incident remain murky.

U.S. forces said they used unmanned drones to follow the insurgents, then fired on them. It was not clear if the jets crossed into Pakistan's airspace.

In a statement, Combined Joint Task Force 101, based in Bagram, said coalition forces used the unmanned aircraft to maintain "positive identification of the enemy" firing at coalition troops.

Pentagon spokesman Bryan Whitman said the operation was coordinated with the Pakistani forces.

The Pakistani army said the coalition airstrike hit a post of the paramilitary Frontier Corps and was a "completely unprovoked and cowardly act."

It launched a strong protest and reserved "the right to protect our citizens and soldiers against aggression," the military said in a statement.

The State Department said Wednesday it had no details of the incident but added that it could "understand" Pakistani anger if innocent lives had been taken.

"Loss of innocent life is obviously a tragedy," said Gonzalo Gallegos, a department spokesman. "It's something we all try to avoid."


Associated Press Writers Matthew Lee and Pauline Jelinek contributed to this report.