KABUL, Jan 21 (Reuters) - Unknown gunmen on Tuesday seized more than 60 Afghan workers clearing Soviet-era anti-tank mines in western Herat province, the largest mass kidnapping undertaken in an economically important area now beset by security problems.

Herat, which borders Iran, has long been plagued by kidnappings. But a sharp downturn in security in the traditionally safe province would be a worrying sign ahead of the departure of most foreign forces by the end of this year.

More than 60 Afghans working for the British demining group The Halo Trust had arrived at a village in Pashtun Zarghun district before 8 a.m. to begin clearing anti-tank mines let over from the 10-year Soviet occupation of the 1980s.

"When they arrived, armed people on motorbikes came and forced them to get into their vehicles and made them drive to a nearby village," The Halo Trust's country director, Farid Homayoun, told Reuters.

The kidnappers forced the deminers out of the cars and marched them towards nearby mountains. They have not been seen since.

"Some deminers were able to escape but over 60 staff are still with those people and at the moment the local shura are in negotiation for their release," Homayoun said, using the Afghan word for council.

Provincial police have been sent to the village. No one has claimed responsibility for the kidnappings.

A small group of deminers was held for a week by Taliban fighters in more volatile southern Kandahar district in April last year before being released.

In Herat, a commercial hub, kidnappers have generally targeted rich business people or their families, with residents staging strikes and protests to draw attention to the issue.

Bombings and insurgent attacks had been infrequent in Herat until recently, but security has deteriorated sharply in recent months. Residents told Reuters it had become dangerous to travel through districts bordering the provinces of Farah and Baghdis. (Additional reporting by Jessica Donati and Hamid Shalizi; Writing by Dylan Welch; Editing by Ron Popeski)