Mount Anak Krakatau, which means Child of Krakatau, belched thick ash over the strait that separates the islands of Java and Sumatra forcing authorities to warn nearby residents to wear masks outside.

"We are still recording continuous eruptions with thick clouds towering at between 500 to 3,000 metres from the peak," Deny Mardiono of Indonesia's Geological Agency told AFP.

Anak Krakatau has erupted at least 21 times in recent weeks but Sunday's eruption was the largest yet, Deny said.

Authorities ordered people to stay out of a two-kilometre exclusion zone around the volcano, which is currently graded at level two of four-tiered volcanic alert system.

"People, including tourists, should adhere to the recommendation from the Geological Agency, which prohibits anyone to be within a two-kilometre radius from the crater," he added.

The volcano has been sporadically active since it emerged from the sea at the beginning of last century in the caldera formed after the 1883 eruption of Mount Krakatau.

That disaster was one of the deadliest and most destructive in history with an estimated 35,000 people killed.

Anak Krakatau last erupted in 2018, generating a tsunami that killed 429 people and left thousands homeless.

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

The country has nearly 130 active volcanoes.