A military helicopter belonging to the NATO-led peacekeeping operation in Afghanistan has crashed in the west of country, killing at least 17 Spanish troops.

Spain's Defense Ministry officials are reported as saying the helicopter went down near the western city of Herat, where Spanish troops have their main military base.

Officials of the NATO-led International Security Assistance Force, ISAF, in Afghanistan say the crash occurred during an exercise. They could not immediately confirm the number of dead.

Major Andrew Elms, a spokesman for the international peacekeeping force in Kabul, rules out possibility of hostile fire causing the crash near Herat airport.

"We do believe that there have been some soldiers killed and there are casualties," he said. "We think it was an accident, so, technical fault. We do not believe that it was enemy activity."

The city of Herat is a relatively secure part of Afghanistan, and guerrillas of the former Taleban regime as well as other militant fighters are less active there than in the eastern and southern parts of the country.

While the cause of the helicopter crash is under investigation, Afghan officials are reported as saying it collided in mid-air with another helicopter.

A Spanish state radio broadcast said that five troops from a second helicopter were injured upon making an emergency landing after seeing the first helicopter crash.

More than 8,000 troops from 36 countries are serving in ISAF, which is led by NATO.

The air disaster is the second for Spanish troops who served in Afghanistan. In 2003, a plane bringing 62 Spanish peacekeepers back from Afghanistan crashed in Turkey, killing all those on board.