KABUL, Afghanistan — Some 2,500 Afghan and U.S.-led coalition troops pushed ahead with a massive offensive in Afghanistan's eastern mountains Thursday to flush out rebels believed hiding there, officials said.

The forces have killed six insurgents since the operation started Tuesday, but there has been no major battle, Afghan Defense Ministry spokesman Gen. Mohammed Zahir Azimi said.

U.S. military spokesman Lt. Mike Cody confirmed the operation in Kunar province's Pech River Valley was going on Thursday and was the biggest offensive this year. He gave no other details.

Taliban-led rebels have stepped up attacks across Afghanistan in the past year, raising concerns for this country's future.

In a statement Wednesday, U.S. Army Command Sgt. Maj. James Redmore said, "We're taking the fight to the terrorists in their own backyard."