"We're assuming they're American," said the official, adding he could not confirm whether those on board had been killed or wounded in the incident.
The official added that poor weather had likely been a factor, but cautioned that nothing was being ruled out.
"The crash site is secured; the cause is under investigation. Additional information will be released as appropriate," NATO's International Security Assistance Force (ISAF) said in a statement.
In NATO terms, southwestern Afghanistan includes Nimruz and Helmand provinces, where nearly 20,000 US Marines are stationed.
While helicopter crashes occur with some regularity in Afghanistan, ISAF says they are rarely the result of Taliban fire.
On March 16, 12 Turkish soldiers and two civilians were killed in a chopper crash in the Afghan capital Kabul.
In January, six US troops were killed in a CH-53 Sea Stallion helicopter crash in the Musa Qala district of Helmand province.
And 30 US troops and eight Afghans were killed in August 2011 when Taliban insurgents shot down a Chinook helicopter, in the deadliest incident for US and NATO forces since the war in Afghanistan began in 2001.
COMMENTS (1)
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If that's the actual photo taken of the incident site, I don't see any sign of "bad weather" in it.