JAKARTA, Sept 12, 2007 (AFP) - Scientists in Indonesia on Wednesday advised people to maintain a five-kilometre (three-mile) safety zone around a volcano that has become more active in densely-populated East Java.

The alert status on Mount Kelud, considered one of the most dangerous on Java, was raised late Tuesday to "beware", the second of a four-level warning system, the vulcanology office said on its website.

The temperature of the water in the crater has been on the rise, while its colour has changed from its usual green to yellow in some areas and a blue-white in others, it said.

A construction worker in the area on Tuesday suffered respiratory difficulties due to the increasing activity at the volcano, the office said, adding that the public should be cleared from a five-kilometre radius.

The volcano's slopes themselves are sparsely inhabited but the peak is a popular domestic tourist destination and is located on a densely-populated plain.

The 1,731-metre (5,712-foot) Mount Kelud last erupted in 1990. Eruptions can be particularly devastating as the water in the crater spilling out creates mudflows known as lahars.

More than 15,000 lives have been claimed by the volcano since 1500, including around 10,000 deaths in a massive 1568 eruption.