KABUL, Afghanistan (AP) – Ten American soldiers died when their transport helicopter crashed during combat operations aimed at flushing out Taliban and al-Qaida militants from remote mountains in eastern Afghanistan, military officials said Saturday.

The crash of the CH-47 Chinook Friday afternoon was the deadliest for US forces here in a year and comes at a time of increasing militant attacks, though US 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 operation, dubbed Operation Mountain Lion, in Kunar province, near the border with Pakistan. It’s one of the biggest offensives since the ouster of the hardline Taliban regime by US-led forces in late 2001 for hosting al-Qaida.

The Chinook was conducting “operations on a mountaintop landing zone” when it crashed near Asadabad in Kunar, about 240 kilometres (150 miles) east of Kabul, the capital, the military said.

The terrain surrounding Asadabad – where the U.S. military has a large base – is extremely rugged, and the police chief of Kunar province, Gen Abdul Ghafar, said the helicopter crashed about 20 kilometres (10 miles) northwest of the base at a remote spot a day’s walk from any passable road.

“The area of the crash is a mountainous area and it is difficult to reach,” said Ghafar.

Recovery operations did not begin until daybreak Saturday.

The 10 deaths brings to at least 25 the number of US military personnel killed in Afghanistan this year, according to the website icasualties.org, which relies on US Defense Department releases for its information.