A NATO helicopter crashed today in Eastern Afghanistan, killing two U.S troops, the third time in 10 days that a coalition helicopter has come down.

The two Americans, who have not been named, were the only crew aboard the OH-58 Kiowa helicopter, according to a Defense Department official.

The cause of the crash is under investigation and officials have not ruled out that the chopper was shot down.

"Sometimes these things take weeks to determine the actual cause," Maj. Adam Wojack of the International Security Assistance Force said.

The incident happened in Logar Province, an area patrolled mostly by U.S forces.

The Taliban quickly claimed responsibility, saying one of their fighters shot the aircraft down. Afghan sources in Logar say a heavy firefight broke out after the helicopter crashed.