Islamabad, Pakistan - Three American troops were killed Wednesday in a bombing in the volatile northwest region of Pakistan, a nation where U.S. military involvement is a highly sensitive matter and where American ground personnel rarely suffer casualties.

The attack targeted forces who were part of a contingent of U.S. military trainers. They were traveling to Lower Dir, a district in North-West Frontier Province that Pakistan's military has said it wrested from Taliban control.

The American military deaths were the first in Pakistan in recent years and shined a light on a joint training program that U.S. and Pakistani military officials have strived to keep quiet.

As many as 100 U.S. Special Forces members are in this country, most of them instructing the weak and poorly equipped Pakistani paramilitary forces battling insurgents in the rugged areas bordering Afghanistan.

The convoy attacked Wednesday was headed for the inauguration of a girls school that had been destroyed by the Taliban and recently restored with U.S. assistance, Pakistani and U.S. officials said.

The blast leveled another new girls school that the convoy was passing. One member of Pakistan's paramilitary Frontier Corps and three schoolgirls were killed, and dozens of people were injured, including two U.S. military personnel and schoolchildren, U.S. and Pakistani officials said. One Pakistani military official, speaking on the condition of anonymity, said the slain Americans were Marines.

The U.S. military has provided counterinsurgency training to Pakistan since 2008, but it is a highly sensitive partnership for both countries. Anti-American sentiment is rising here, fueled by media reports and public anger over perceived slights to Pakistan's sovereignty by the United States. Pakistan does not allow U.S. combat operations on its soil, and while it privately condones CIA missile strikes targeting insurgents in the tribal areas, it publicly condemns them.

Wednesday's bombing seemed destined to incite more animosity toward the United States.

The Pakistani Taliban, which has carried out a spate of attacks on civilian and security targets in recent months, asserted responsibility in a statement to news services. It said the slain U.S. personnel were employees of the American security company formerly known as Blackwater, a frequent subject of suspicion and scorn among segments of the Pakistani media.

In Washington, Richard Holbrooke, the U.S. special representative to the region, dismissed that assertion as "propaganda."

The U.S. Embassy in Islamabad, the capital, said in a statement that the American military personnel were in Pakistan to "conduct training at the invitation of the Pakistan Frontier Corps. ... The United States and Pakistan are partners in fighting terrorism - and our people are working together to build schools."

The training program is controversial within Pakistan's military, said Mahmood Shah, a security analyst and retired army general. It gives "a reason for the Taliban to fight the Pakistan army," he said, adding that many military officials here believe the Americans have little to teach about counterinsurgency.

U.S. officials have said the training program is evidence of improved relations and intelligence-sharing with Pakistan. Navy Adm. Eric Olson, head of U.S. Special Operations Command, told a Senate committee last year that American troops participating in the program were "learning much about our common adversaries and the social complexities of the region."

Wednesday's bombing raised the "very serious" possibility that members of the Pakistani forces had informed militants about the location of the convoy, a Pakistani security official said, speaking on the condition of anonymity. The Frontier Corps is drawn largely from the same Pashtun population that lives along the Afghan border and feeds the Taliban.

To witnesses, the blast demonstrated the militants' continued sway in the region.

"It is unfortunate that such brazen acts of terrorism are taking place despite tall claims of the security forces that they have cleared the area of the militants," said Haji Tahir Khan, 68, a local elder.

Khan, a special correspondent, reported from Peshawar. Special correspondent Shaiq Hussain in Islamabad contributed to this report.