Two U.S. Marines were killed in southern Afghanistan and a civilian helicopter went down in Helmand province, officials said.

Brig. Gen. Eric Tremblay, spokesman for NATO's International Security Assistance Force, said the two Marines were killed Monday but released no other details about their deaths, CNN reported.

A NATO representative and local police, meanwhile, said Tuesday there were no immediate reports of casualties in the helicopter crash, but CNN reported a Taliban spokesman said the militants shot down an aircraft and killed 37 foreign soldiers. CNN said such claims by the Taliban frequently turn out to be exaggerated or false.

The incidents came as nearly 4,000 U.S., NATO and Afghan troops were sweeping through Helmand province to clear the one-time Taliban stronghold of militants, joining an earlier push by British forces.