"Increased underground magma movement has built up pressure on the blocked entrance, causing the bottom of the lake to rise and crack," Saut Simatupang, an official at the Centre for Volcanology and Geological and Hazard Mitigation, told Reuters.

Authorities have been monitoring the 1,731-metre (5,700-foot) volcano for several weeks and raised its alert status to the highest level about two weeks ago as its activity increased and an eruption appeared imminent.

An estimated 350,000 people live within 10 km (6 miles) of the volcano, which is about 90 km southwest of Surabaya, Indonesia's second-largest city and one of its busiest airports.

The deadly volcano spewed ash about 500 metres into the air on Sunday, a day after confusion over whether it had already started erupting.

Officials at the vulcanology centre said on Saturday that the volcano had erupted while hidden by heavy cloud cover, but later they said that an eruption had not in fact taken place.

Scientists have suggested that hardened lava from previous eruptions could be blocking the release of magma, and warn this could burst out once sufficient energy has built up.

A scientist monitoring the volcano said that authorities have stopped measuring the water temperature in the crater lake since Sunday afternoon because of the damaged equipment.

"We stopped measuring the temperature of the crater lake after our equipment broke," said Umar Rosadi, the scientist monitoring the volcano's activity.

"We are now left with four seismic quake detectors and two deformation detectors."

Kelud, also known as Kelut, means "sweeper" in Javanese, a reference to the fact that when it erupts, it sweeps away everything in its path.

When it last erupted in 1990 at least 30 people were killed, while about 5,000 died in 1919 when it spewed scalding water from its crater lake.