Mount Semeru in eastern Java erupted on Wednesday afternoon, throwing ash and gas more than 13 kilometres (eight miles) 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 stayed in 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.

Resident Faiz Ramadhani told AFP the eruption had been "very horrible."

"At that time, four o'clock in the afternoon, it was like midnight. It was very dark," the 20-year-old said.

Some houses near the volcano were partially buried by volcanic ash and rock fragments.

Nurul Yakin Pribadi, head of Supiturang village, said he was "shocked" to find his house was damaged.

"There was a metre-high spill of (volcanic) materials on my house," he told AFP. "Many people's houses were damaged."

At least one elementary school was flattened, Lumajang district secretary Agus Triyono said, adding that authorities were still gathering data on the damage to infrastructure.

At least three people suffered from burns, a search and rescue official said in a statement.

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.

An eruption by Semeru in 2021 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.