At least three hikers were killed when one of Indonesia’s most active volcanoes erupted on Friday, spewing ash miles into the air, officials said.

Mount Dukono, on Halmahera Island in eastern Indonesia, erupted around 7:40 a.m., causing a booming sound and sending a thick ash cloud about six miles above the summit, according to the Volcanological Survey of Indonesia.

Officials said Friday afternoon that volcanic activity there was increasing and that hikers had been banned from climbing the volcano since last month.

Rescuers scrambled to look for the 20 people who were on the volcano when it erupted, Erlichson Pasaribu, a local police chief, said in an interview.

He said three of the hikers — two Singaporeans and an Indonesian national — were later confirmed dead. Fifteen others had climbed down safely and two porters who were part of the original hiking group were helping to retrieve the bodies, he said.

Mount Dukono has been active for decades, erupting continuously since 1933. Over the past month or so, it erupted nearly 200 times, the Volcanological Survey of Indonesia said.

Mr. Erlichson, the police chief of North Halmahera, the regency where the volcano is located, said that seven of the hikers who climbed down safely were Singaporean and the other eight were Indonesian.

The hikers were aware that climbing the volcano was banned because the authorities had sent an alert on April 17 warning that it was active, Mr. Erlichson said. He said he would be questioning the hikers’ tour guides about how the hike went ahead despite the restriction.

The thousands of islands that make up the Indonesian archipelago sit along the Ring of Fire, a seismic zone encircling the Pacific Ocean. The country had about 127 active volcanoes as of last year, according to the Geological Agency of Indonesia.