A US Black Hawk helicopter crashed in southwestern Afghanistan on Thursday with four people believed to be on board, likely American soldiers, a US defense official told AFP.
"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.
The crash came a day after another difficult blow to the US-led war effort in Afghanistan -- the publication of photos showing US troops abusing the mangled remains of Taliban insurgents.
Afghan President Hamid Karzai on Thursday called for an "accelerated" transition of security responsibilities from NATO forces in the wake of the scandal, the latest in a series involving US troops.
Those incidents have damaged Afghan-US relations and fueled anti-Western sentiment in the war-wracked country.
NATO has a 130,000-strong military force fighting the Taliban, which has led an insurgency against the Western-backed Kabul government since being toppled from power by a 2001 US-led invasion.
Afghan forces are gradually taking over control of security in the country, with the goal of being in the lead nationwide next year and enabling most foreign troops to depart by the end of 2014.