Islamabad, Pakistan (CNN) -- A "vicious" roadside bomb blast struck a convoy in northwestern Pakistan, killing at least seven people Wednesday, including three American troops training the country's security forces.
The powerful blast rippled through Lower Dir in the North West Frontier Province. The region is the site of frequent attacks by militants retaliating against an army offensive trying to rout them from their havens along the Pakistan-Afghanistan border.
"This is guerrilla warfare here in Pakistan," a Pakistani military official told CNN. "Unfortunately, I don't see it ending anytime soon. It's going to continue for years."
Pakistani military spokesman Maj. Gen. Athar Abbas said the other fatalities were a Pakistani soldier and three schoolgirls. The blast also wounded 120 people, said Dr. Waqeel Mohammad, administrator of a hospital in the district.
When the blast occurred, the Americans were part of a convoy heading to the reopening of a girls school rebuilt by the Pakistani military after militants destroyed it in an earlier strike.
The vehicle carrying the three American troops bore the impact of the bomb, which police say was remotely detonated, and the powerful blast damaged a nearby girls school -- not the one the convoy was headed to, which was 8 miles away.
The U.S. Embassy in Islamabad said two of the wounded were U.S. military service members and added that children were among the Pakistani casualties.
The U.S. troops are among a group of about 70 U.S. military personnel that have been training Pakistani troops. The Americans are there at the invitation of the Pakistan Frontier Corps -- drawn from tribes in the border area and considered vital in the fight against militants. They have been teaching the corps counterinsurgency skills critical to fighting the Taliban and al Qaeda and how to train their own trainers.
A U.S. military official told CNN the dead and wounded U.S. troops were Special Operations forces. They're part of a "civil affairs" team that provided training in "hold and build operations" such as renovating schools and providing local aid, which is why they have been at the school, this official said.
A U.S. Central Command statement said the troops were assigned to the Office of the Defense Representative, Pakistan, to conduct what it called "civil affairs-related training."
"This attack demonstrates the terrorists' lack of respect for life, and their willingness to use violence against women and children as a means for advancing their malign vision," said Rear Adm. Hal Pittman, Central Command spokesman.
"The U.S. personnel were in Pakistan at the request of the Government of Pakistan to assist the Pakistanis with training in support of our long-standing partnership with Pakistan, and this horrific attack will not dissuade that partnership. We extend our sincere condolences to the families and loved ones of those innocent individuals who were killed or injured."
A U.S. Embassy statement said the United States deplored "this vicious terrorist bombing."
"We express our condolences to the families of the deceased and our sympathy and support to the wounded," it said.
Muhammad Israr, a correspondent for the Pashto-language TV channel AVT Khyber, called the scene "horrible."
"We saw the corpses of the foreigners in the wreckage of their car," he said.
The school celebrating its reopening was rebuilt by the Pakistani military as part of a project funded by the U.S. Agency for International Development, a government entity that provides economic and humanitarian assistance worldwide.
It is unclear whether the insurgents knew American troops would be going to the event and whether U.S. military trainers were specifically being targeted.
A senior Pentagon official said that angle is being explored even though there is no evidence they were targeted. This source said attending such an event wasn't "part of their normal duties," but it is possible they might have been using the occasion to train Pakistanis on how to conduct security for similar events.
The United States wants to send more personnel to Pakistan to supplement the work of the 70 people there, but the Pakistanis have declined that help.
CNN's Reza Sayah, Barbara Starr and Kiran Khalid and journalists Umar Aziz Khan and Nasir Habib contributed to this report.