Jakarta - The offspring of the Krakatoa volcano in the Indonesian Strait of Sunda has increased its activity and prompted authorities to raise its alert status to keep visitors away, volcanologists said Thursday.

Tremors from Anak Krakatau have been on the rise since early this month, and it spewed volcanic debris as far as 700 metres from its crater, experts said.

Surono, head of Indonesia's Centre for Volcanology and Geological Hazard Mitigation, said Anak Krakatau on Tuesday produced up to 230 tremors and spewed black smoke up to 1,000 metres into the air.

Surono, who like many Indonesians goes by only one name, said volcanologists on Wednesday raised the volcano's alert status and declared it a no-go zone.

"Residents, fishermen and tourists have been warned to stay 2 kilometres away from Anak Krakatau," he said in a statement.

Anak Krakatau, or "Child of Krakatoa," is located in the Sunda Strait between Java and Sumatra islands. An eruption in June 1994 killed one US tourist and injured three Britons and two Indonesians.

Krakatoa exploded in August 1883 in one of the world's greatest recorded eruptions. The sound of the blast was heard in Australia, 3,500 kilometres away. The volcano generated a 36-metre tidal wave that crashed onto nearby islands, and 36,000 people were killed by the water and ash.

Indonesia has the highest density of volcanos in the world with about 500 along the 4,800-kilometre archipelago. Nearly 130 are active, and 65 are listed as dangerous.