Two U.S. Marines were killed in a "hostile incident" in southern Afghanistan, where a white transport helicopter also crashed, officials said Tuesday.

The two Marines were killed in the southern Helmand province on Monday, said U.S. military spokeswoman Capt. Elizabeth Mathias. She did not have other details.

Some 4,000 Marines are pushing through Helmand province in the biggest U.S. military operation in Afghanistan since the ouster of the Taliban from power in 2001. The region is the world's largest opium poppy producing area and the Taliban's heartland.

Taliban fighters have planted dozens of roadside bombs in the region, one of the greatest threats to troops operating there. Militants have increased their attacks dramatically in the last three years.

The deaths bring to 107 the number of U.S. troops killed in Afghanistan so far this year, compared with 151 U.S. deaths in all of 2008. As of Monday, at least 660 members of the U.S. military had died in Afghanistan, Pakistan and Uzbekistan since 2001, according to the Defense Department. Of those, the military says 492 were killed by hostile action.

Elsewhere in Helmand, a white transport helicopter crashed around daybreak Tuesday in Sangin district, said Fazel Haq, the top district official.

It was not clear if there were casualties, Haq said. The helicopter caught fire.

The helicopter is civilian, not military, but it wasn't immediately clear who it belongs to, said U.S. military spokeswoman Lt. Cmdr. Christine Sidenstricker.

White transport helicopters are used by private contractors and organizations such as the U.N. and the U.S. Embassy, but U.N. and U.S. officials said the helicopter did not appear to be one of theirs.

Separately, a roadside blast Tuesday hit a civilian vehicle in Uruzgan, another southern province, killing three people and wounded six others, according to an Interior Ministry statement. It said all the victims were civilians.