Initial reports from NATO's International Security Assistance Force indicated the six died Tuesday in southern Afghanistan after their Black Hawk helicopter crashed.
However, two U.S. officials told CNN the Pentagon is looking into whether the troops came under enemy fire after surviving the chopper's crash or died in the crash itself.
One of the officials told the U.S. broadcaster: "It's unclear if it was the crash or contact with the enemy after the helo came down."
Stars and Stripes, quoting a U.S. defense official, reported there was one survivor in the Black Hawk UH-60 incident, while another six died. Other details were not available.
The ISAF said initial reports indicated there was no enemy activity in the area. They did not confirm the exact location of the crash, but some media reports said it was in Zabul province near the border with Pakistan.
CNN said military officials initially thought mechanical failure was likely, but then questions came up whether the troops had survived the crash only to come under mortar fire.
"We believe there was some sort of enemy engagement once the helicopter crashed," one of the two officials told CNN. But the two also stressed there had been no conclusion about the report of enemy fire.
Defense officials planned to interview crew members of a second helicopter that was in the area at the time. Investigators also would probe the wreckage and conduct autopsies for clues.
It was the single-deadliest day for the U.S. troops in Afghanistan since August of last year when seven Americans and four Afghans died in a helicopter crash, CNN said.
Prior to Tuesday, there had been 156 coalition forces killed so far this year in Afghanistan, iCasualties.org said.
Britain's Guardian newspaper said military casualties have been coming down with NATO and the United States withdrawal troops from Afghanistan. There were more than 400 coalition forces killed last year.
The troop withdrawal will be completed by the end of next year, leaving Afghan forces to take full charge of their country's security.