Six U.S. Marines died when their helicopter went down in southern Afghanistan's Helmand province around midnight local time on Thursday.

There were no signs of enemy activity in the crash of the U.S. Marine CH-53 Sea Stallion, NBC News reported, but an investigation is underway.

Until the families of the Marines are notified, authorities would not provide any more details.

Last August, 30 U.S. troops, including 22 elite Navy SEAL commandos, were killed when their helicopter crashed in eastern Afghanistan.