The eruption at Dukono on the island of Halmahera produced a cloud of smoke that rose about one mile into the air.
There are three dead: two foreigners and one resident of the island of Ternate, police chief Erlichson Pasaribu told the television channel.
Several of the roughly 20 people who were initially reported missing were reported to be from Singapore.
So far, 15 climbers have descended to safety, the police chief said.
The group's guide and a porter could face charges for taking hikers into a prohibited area, the police chief added.
