"Kelud is now on the point of no return."

The number of volcanic earthquakes at Mount Kelud, 90 km (55 miles) southwest of Indonesia's second-largest city Surabaya, has risen to as high as 23 in one day, compared with a maximum of 15 a day just before its last eruption in 1990, Surono said.

The 1,731-metre (5,712-foot) volcano's "deformation" or expansion has increased, and gas and chemical levels have risen, while the temperature of the lake in the volcano's crater is climbing more rapidly, hitting 37.4 degrees Celsius on Saturday, compared with 32 degrees in August.

Surono said he has a text message on his mobile phone ready to send out to government and rescue officials if he decides to put the volcano in East Java on the highest state of alert ahead of a likely eruption.

He and his team in Bandung, a city in West Java which is circled by volcano peaks, have been monitoring Mount Kelud for weeks, after three other volcanoes erupted earlier this year in Indonesia.

When Kelud last erupted in 1990, at least 30 people were killed, while an eruption in 1919 killed about 5,000 as it ejected scalding water from its crater lake.

CRATER LAKE PROBLEM

The crater lake is one reason Kelud is harder to monitor than many of the other volcanoes scattered across Indonesia's archipelago, as some of the warning signs are concealed below the water's surface, Surono said.

"It's very difficult to observe. We cannot see the crater or the small eruptions before the main eruption," he said.

The crater, now closed to the public, is monitored by CCTV so scientists can detect any change in the lake's colour, likely to turn from turquoise to white as sulphur levels increase.

"Whether this will be followed by eruption in the near term is very difficult to say," Surono said.

The decision to raise the alert is a tough one, he said, because of the likely economic impact on local residents.

Local people have been told to stay out of a 5 km radius of the volcano, but an estimated 350,000 live within 10 km, growing coffee, sugar cane, pineapples and papayas on the rich volcanic soil or feeding their cattle on the volcano's slopes, and some are ignoring the warnings, Surono said.

Local authorities have taken precautions, ensuring channels and barriers to protect the area from lahar, a mix of hot stones and water, have been strengthened.

Some villagers who survived the 1990 eruption have said they plan to shelter under tables and tarpaulins rather than leave their homes.

"Many people experienced this volcano, and they say they will stay in their houses because they are not scared," said Surono, although he fears such measures would prove inadequate if Mount Kelud showers the area with hot ash.

Indonesia, which sits on a belt of intense seismic activity known as the Pacific Ring of Fire, has had several volcanic eruptions over the centuries.

The 1815 eruption of Mount Tambora on Sumbawa killed about 90,000 people and the material thrown up blocked the sun's light for months, while the eruption of Bali's Mount Agung in 1963 killed about 1,500 people.