A US Black Hawk helicopter that Washington said crashed in Afghanistan with four people believed to be on board had been responding to a suicide bombing, an Afghan official said on Friday.
A US defense official in Washington told AFP that those on board were likely American soldiers, but could not confirm whether they had been killed or wounded in Thursday's incident in the country's southwest.
A senior police officer in Helmand province said the helicopter went down in stormy weather on a flight related to a deadly suicide attack on an Afghan police post in the province's Garmser district.
"There was a suicide attack on a police checkpoint that killed four police and wounded seven others," he said, adding that it was unclear whether the helicopter was heading to the area in support or to pick up casualties.
"There is no proof of Taliban involvement in the crash," Mohammad Islamil Hotak told AFP.
He said police had received initial reports that all four members of NATO's International Security Assistance Force (ISAF) on board had been killed in the crash, but that this had not been confirmed.
The US official said poor weather had likely been a factor in the incident, 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," ISAF said in a statement.
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.