Anak Krakatau has lost about two-thirds of its height and three-quarters of its mass since an eruption on Dec 22 that triggered a deadly tsunami, Indonesia's national geological agency said at a media briefing yesterday.

Based on photographs taken of Anak Krakatau, researchers confirmed that the huge landslides took place following the Dec 22 eruption and subsequent eruptions, causing its height to drop from 338m to 110m, said Mr Antonius Ratdomopurbo, the agency's secretary.

The volcano, whose name means "child of Krakatau", lies in the Sunda Strait separating Java and Sumatra islands.

A section of its slope collapsed after the Dec 22 eruption and slid into the ocean, which displaced massive amounts of water, generating giant waves of up to 5m high that inundated the coastlines of Java and Sumatra.

The Anak Krakatau volcano now has a mass of about 50 million cubic metres, or about 20 to 25 per cent of its original size, Mr Antonius said.

He said the agency would continue to monitor the volcano's activities in the coming days before reviewing its current alert status.

Last Thursday, the danger alert for Anak Krakatau was raised to three, the second highest on the four-level scale.

A no-go zone around it has been expanded from 2km to 5km.

"There is now a small potential for Anak Krakatau to trigger another tsunami," he said.

The volcano has been rumbling on and off since July but was particularly active for a few days up to late last Friday, spewing columns of ashes of up to 3,000m which ended up falling on nearby cities. This came even as aid workers and rescuers struggled to reach communities reeling from the devastation wreaked by the killer waves.

As at yesterday, 431 people have been killed, with more than 7,000 others injured and another 15 still missing, in the third major natural disaster to hit Indonesia this year following earthquakes in Sulawesi and Lombok.

Wahyudi Soeriaatmadja