A large-scale helicopter-borne assault into a remote, insurgent-held sanctuary near the border with Pakistan left six U.S. soldiers dead in heavy fighting with Afghan and Pakistani insurgents, U.S. officials said.

The operation, which was continuing Thursday, was designed to drive back the enemy in the remote and mountainous border region. One Afghan soldier was also killed in the assault.

The deaths show the difficulty U.S. troops have faced in stemming the violence in Konar province, which has been one of the most violent regions of Afghanistan since 2005. The current U.S. strategy, which focuses on major population centers, has led the U.S. military to shift emphasis away from the province and pull some troops out of its remote valleys.

Violence levels throughout Konar remain among the highest in Afghanistan.

The area in eastern Konar province where the six soldiers were killed has long been a problem for U.S. forces because of its proximity to the largely ungoverned regions of Pakistan's tribal areas. In June, about 600 soldiers from the battalion involved in this week's operation killed about 150 fighters in the same valley. Two U.S. soldiers were killed in that assault.

After the summer operation, U.S. commanders tried to establish an Afghan police station in Daridam, one of the main villages in the remote valley, but the Afghan police abandoned the area when U.S. troops returned to their nearby bases.

"We built them a station," Lt. Col. J.B. Vowell, the local commander, said in an interview late last year. "We slung in a container, cut windows in it and surrounded it with barriers and sandbags. The police were too scared the Taliban were going to come back and kill them. . . . The people are still timid, and the police are timid."

In this week's assault, U.S. forces pushed deeper into the valley and closer to the Pakistan border than they had in years, killing large numbers of enemy fighters and uncovering several significant weapons caches, a U.S. military official said.