Kabul, Afghanistan ? Ten U.S. soldiers died when their helicopter crashed during combat operations aimed at flushing out militants from remote mountains in eastern Afghanistan, officials said Saturday.

The crash of the CH-47 Chinook on Friday afternoon was the deadliest for U.S. forces here in a year and comes at a time of increasing militant attacks, though U.S. officials ruled out hostile fire as a cause.

“There is no indication that the helicopter came down due to some enemy action,” Lt. Tamara D. Lawrence, a coalition spokeswoman, told The Associated Press.

Some 2,500 Afghan and U.S. soldiers are conducting a joint military campaign, dubbed Operation Mountain Lion, in Kunar province near the border with Pakistan. It is one of the biggest offensives since the ouster of the hard-line Taliban regime by U.S.-led forces in late 2001 for hosting al-Qaida.

The transport helicopter was conducting “operations on a mountaintop landing zone” when it crashed near Asadabad in Kunar, about 150 miles east of the capital, Kabul, the military said.

The terrain surrounding Asadabad – where the U.S. military has a large base – is extremely rugged. The police chief of Kunar province, Gen. Abdul Ghafar, said the helicopter crashed about 10 miles northwest of the base at a remote spot a day’s walk from any passable road.