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A large-scale helicopter- borne assault into a remote, insurgent-held sanctuary near the Afghan 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 also was killed.

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 area has been a problem for U.S. forces because of its proximity to the largely ungoverned regions of Pakistan’s tribal areas. Last 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 valley, but Afghan police abandoned the area when U.S. troops returned to their nearby bases.

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.