Four US troops die in Afghan chopper crash: NATO
Two other soldiers were injured in what the NATO-run International Security Assistance Force (ISAF) said was believed to have been a mid-air collision in the south, a Taliban heartland and the most volatile part of Afghanistan.
"Four ISAF service members were killed and two others injured in the incident," it said in a statement.
"The incident is currently being investigated, but it is confirmed that hostile fire was not involved," it said.
The current year has been the most deadly for the more than 100,000 NATO and US troops fighting a resurgent Taliban since the 2001 US-led invasion ousted the Islamists' extremist regime in Kabul.
The independent icasualties.org website, which keeps a running tally of foreign troops deaths in Afghanistan says that so far in 2009, 424 have died. Of those, 256 were American, icasualties says.
Colonel Wayne Shanks, US military spokesman in Kabul, confirmed the four dead soldiers were Americans.
Another helicopter came down in western Afghanistan -- also in the early hours of Monday, an ISAF spokesman said.
"Military casualties are reported and a recovery operation is underway," the statement said.
It said the helicopter came down "for unconfirmed reasons" after a joint Afghan-international security force "searched a suspected compound believed to harbour insurgents conducting activities related to narcotics trafficking".
"During the operation, insurgent forces engaged the joint force and more than a dozen enemy fighters were killed in the ensuing fire fight," it said.
It did not give the nationality of the foreign troops involved.
Western Afghanistan has become increasingly hostile in recent months as the Taliban spreads its tentacles across the country.
The London-based think tank International Council on Security and Development, has estimated that the insurgent group has a permanent presence across 80 percent of Afghanistan.
As the country prepares for a run-off presidential election on November 7, the Taliban has threatened to repeat the violence and intimidation that kept millions of people away from the polls the first time round on August 20.
Tensions are rising ahead of the vote -- which will pit President Hamid Karzai against former foreign minister Abdullah Abdullah -- with anti-Western demonstrations in Kabul on Sunday.
Officials have said that arrangements for election security should be finalised on Monday, with Afghan police and army backed by NATO forces aiming to secure polling stations across the country as safe for voters.