Whoever was to blame, the U.S. airstrikes that may have killed friendly fighters in Pakistan have inflamed relations between the countries and could undermine the struggle to stem violence along the Afghan border.

The bombings fueled anti-U.S. sentiment in Pakistan and raised fresh questions about cooperative efforts against suspected terrorists in the lawless region. This is the area, American military leaders believe, that could spawn a major attack against the United States.

Little was certain about what happened.

U.S. diplomats offered apologies for the reported casualties. The Pentagon insisted surveillance drones tracking the bombings showed they hit exactly their intended targets: about a half-dozen enemy fighters firing on coalition forces.

Defense Department press secretary Geoff Morrell said it was too early to know whether the strike killed 11 Pakistani paramilitary forces, as Pakistan alleged.

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

The incident has fed suspicions about U.S. military operations inside Pakistan, as well as about Pakistan's inability to control Taliban or al-Qaida terrorists in safe havens along the border.

The Pakistan's new government has tried to broker a peace deal with tribal leaders in the region. U.S. officials have expressed skepticism about the plan, and there have been questions about Pakistan's commitment and ability to wage a counterinsurgency battle.

The U.S. has promised to send 20 to 30 trainers to instruct Pakistani officers, who will train some 8,500 border Frontier Corps troops this summer, the chairman of the Joint Chiefs of Staff told reporters Tuesday, before the incident. Adm. Mike Mullen also agreed there is some merit in the negotiations that could peel off tribal leaders who would become allies while isolating extremists.

"I am learning as I go that these tribal areas are extraordinarily complex. There's no simple answer," Mullen said. He added that the U.S. wants peace agreements that can be enforced to prevent insurgents from crossing the border.

Rick Barton, a Pakistan expert at the Center for Strategic and International Studies, said the incident had come with Pakistan's government already overwhelmed and trying to find its way.

"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 diplomatic strains were immediately apparent.

In Pakistan, U.S. Ambassador Anne Patterson was summoned to the Foreign Ministry, where the government 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. State Department spokesman Gonzalo Gallegos regretted the loss of lives, but said, "our troops were defending themselves from a hostile act, which they have a right to do."

He said the incident "is a reminder that better cross-border communications between forces is vital."

Military officials said the bombings were under investigation. The Pentagon did not rule out the possibility that friendly forces were killed, but officials did not discount the idea that paramilitary fighters may have attacked coalition troops.

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

The U.S. military said enemy fighters had begun firing on coalition troops about 650 meters inside Konar province. U.S. forces returned fire and also used unmanned drones to follow the insurgents.

As the drones watched, two F-15 fighters and a B-1 bomber launched about a dozen bombs on the enemy fighters who had crossed into Pakistan, U.S. military officials said.

In a statement, Combined Joint Task Force 101, based at Bagram Air Field in Afghanistan, said coalition forces used the unmanned aircraft to maintain "positive identification of the enemy" firing at them. The statement said that the operation was coordinated with the Pakistani forces.

Morrell, the Pentagon spokesman, said the U.S. hopes any peace deal between the Pakistanis and tribal leaders is enforceable so the region does not continue to be a safe haven for al-Qaida.

The U.S. and Pakistan have "a vitally important relationship in an extremely dangerous part of the world," said Morrell. "It is incumbent upon both of us not to let an incident like this or any other interfere with that fundamental shared goal of making sure the FATA (Pakistan's Federally Administered Tribal Areas) is not a refuge for terrorists who may be plotting attacks against the Pakistani government, the United States government, or any of our allies."

As recently as Monday, Mullen said that planning for the next attack against America is going on among insurgents in the border region.

"I'm not saying it's guaranteed it will happen, or that it is imminent," said Mullen, who has visited Pakistan three times since February. "We know that planning is taking place. ... That is a threat to us that must be dealt with."