Two other people were reported missing and eight were trapped in debris, authorities said.

More than 300 families with children took shelter in badly hit Curah Kerobokan village in Lumajang in East Java after their houses were destroyed by volcanic ash and lava.

Residents in other nearby villages fled in panic as cold lava and volcanic ash reached their neighbourhoods, local authorities said.

Rescuers in Curah Kerobokan found a body after a one-hour search.

Lumajang deputy regent Indah Masdar said 41 people suffered serious burns after being hit by hot lava.

"We brought the injured to health centres and for those whose burns are very serious were rushed to hospitals," Masdar said.

Two of the injured being treated at health centres were pregnant women, she added.

Thick debris and knee-level cold lava floods made it difficult for rescuers to search for the two missing locals and to evacuate eight people trapped in a building covered by mud.

Access to Lumajang was also cut after cold lava flow destroyed a bridge, the deputy regent said.

The national mitigation agency said it has dispatched a team to help people at shelters but thick debris from volcanic ash had prevented them from reaching the destination.

The Indonesian Meteorology and Geophysics Agency said its satellite images show volcanic ash has entered the Indian Ocean in the southern part of Java island.

A video released by the agency showed locals, including scores of children, running for safety as Mount Semeru, Java's highest mountain at 3,676 m, erupted at 3:00 pm local time (0700 GMT) on Saturday.

Semeru's alert status has remained at its second-highest level since its previous major eruption in December 2020, which also forced thousands to flee and left villages covered.

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

The Southeast Asian archipelago nation has nearly 130 active volcanoes.

In late 2018, a volcano in the strait between Java and Sumatra islands erupted, causing an underwater landslide and tsunami which killed more than 400 people.