A helicopter belonging to the North Atlantic Treaty Organisation-led international security force in Afghanistan crashed on Tuesday, killing an unspecified number of foreign troops, a spokeswoman for the force said.

Rescuers have reached the site of the crash near the western city of Herat, but it was not clear how many casualties there were, said Squadron Leader Andie Tolan, a spokeswoman for the International Security Assistance Force.

"ISAF soldiers have been involved and some have been killed," she said. "We don't know how many people were on board and why it crashed."