US Marines, Afghan special forces fighting militants in mountains

Hundreds of US Marines and Afghan special forces are involved in a major operation in a remote and mountainous area of Afghanistan.

They are trying to retake a valley controlled by militants suspected of ambushing a team of US commandos and shooting down a special forces helicopter.

The offensive in Kunar province near the Pakistani border is the biggest yet against those believed responsible for the twin attacks on June 28, Sky News reports.

They were the most deadly blow delivered against US forces in Afghanistan since the ousting of the Taliban in 2001.

Three members of a four-man Navy SEAL team were killed in the ambush.

And all 16 troops on board the Chinook helicopter that was sent to rescue them died when it was hit by a rocket-propelled grenade.

"We want them running for their lives way up in the hills," said US Marine Captain John Moshane.

"We want to isolate them from the community."