Twelve women working in an illegal gold mine in Indonesia's North Sumatra province were killed when a cliff collapsed and triggered a landslide that buried them, police have said.

The women, aged 30 to 55 years old, where digging for leftover gold in an abandoned illegal mine in Mandailing Natal district when the accident happened on Thursday afternoon.

"The cliff around the mine collapsed and buried the 12 women, killing them all," local police chief Marlon Rajagukguk told AFP late Thursday, adding that the victims were not professional miners.