Jakarta  - Indonesian authorities on Tuesday raised the alert status for the offspring of the Krakatoa volcano in the Strait of Sunda, and Mount Ibu in eastern province of North Maluku, after both showed increased activity.

Anak Krakatau, or the "Child of Krakatoa," in the Sunda Straits between Java and Sumatra, and Ibu on the Halmahera island of the North Maluku province, have began spewing ash and sending out volcanic tremors in recent days. Volcanology experts raised the alert status to level two.

No evacuation order was given, but volcanologists appealed the villagers living nearby Ibu volcano to wear face-masks to protect against ash.

The directorate general of volcanology said in its website that people had been ordered to stay outside a radius of two kilometres from the volcanos' craters.

Anak Krakatau's latest big 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 as far as Australia, about 3,500 kilometres away. The volcano generated a 36-metre tidal wave that crashed onto nearby islands and killed an estimated 36,000 people.

The 1,340-metre Mount Ibu volcano, about 2,700 kilometres north-east of Jakarta, had a small explosive eruption in 1911. No human casualties were reported.

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. (dpa)