Hundreds of Indonesians were staying in temporary shelters after a volcano on the main island of Java erupted, with officials evacuating around 190 people from its slopes, authorities said.

Mount Semeru in eastern Java erupted on Wednesday afternoon, throwing ash and gas more than 13 kilometres away and forcing officials to raise the alert status to its highest level.

Volcanic activity had largely calmed down but was still fluctuating on Thursday, according to the Indonesian geological agency.

Nearly 900 people were moved to shelters set up in schools, mosques and village halls after the eruption, disaster agency official Sultan Syafaat said.

"During the night, they stay (in shelters) probably because they are still traumatised," he said.

Authorities were also evacuating nearly 190 people from the volcano's slopes on Thursday, most of whom were hikers who had been stranded at a campsite after the eruption, said Rudijanto Tjahja Nugraha, head of the Semeru national park.

When Semeru erupted in 2021, it killed more than 50 people and damaged more than 5,000 homes, forcing almost 10,000 people to seek refuge.

Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes substantial volcanic and seismic activity.

The Southeast Asian archipelago has nearly 130 active volcanoes.