A second helicopter also made an emergency landing and an unspecified number of troops on board that one were believed to be injured, said Maj. Andrew Elmes, a spokesman for the International Security Assistance Force in Kabul.
He said the cause of the crash and the emergency landing — both in the desert south of the western city of Herat — were believed to be mechanical failure. He did not elaborate.
However, Afghan army commander Abdul Wahab Walizada, whose troops are providing security in the area, said the two choppers came too close while flying and the rotor blade of one hit the other.
In Madrid, a Spanish defence ministry official said 12 soldiers and five crew died in the accident, but that the cause was unknown.
The Spanish Prime Minister Jose Luis Rodriguez Zapatero broke off his vacation in the Canary Islands to return to Madrid.
In the Netherlands, a Dutch Air Force official speaking on condition of anonymity said 13 were dead, five were injured, one was missing and that the helicopter was Spanish.
Maj. Elmes declined to comment on the nationality of the troops or how many casualties there were. He said rescuers were on the site, recovering the dead and wounded.
"We do not think the helicopter crashed because of enemy activity. We think it was an accident. We think it crashed into the ground after a mechanical failure, but we are not sure and we are investigating," he said. "The second helicopter landed heavily. There are survivors from that helicopter."
The NATO force includes about 10,000 troops from 36 nations, which maintains security in the capital, Kabul, and the country's north and west. It plans to increase its size by an unspecified amount and take over from the US-led coalition in the violence-wracked south early next year, before gradually moving into the east.
In June, suspected insurgents shot down a U.S. military Chinook helicopter in eastern Kunar province — a hotbed for Taliban and al-Qaida insurgents — near the border with Pakistan. All 16 U.S. forces on board were killed.
In April, 15 US service members and three American civilians were killed when their Chinook went down in a sandstorm while returning to the main US base at Bagram.Reuse content