But geologists say a wider area could be affected by subsidence as a result of the phenomenon and could see more people forced from their homes.

"The latest data, from January, showed that there is an elliptical area of about 1.5 kilometres (about one mile) wide and 3.0 kilometres long ... that may suffer from subsidence because of the mud outflow," said geologist Adang Bachtiar.

Some places had already sunk by up to one metre (yard), he added.

The wider area covers the main mud crater and areas the mud has yet to reach, but the geologist said it was still too early to order an evacuation.

Workers at the site of the mud crater, which lies near Indonesia's second largest city of Surabaya, are trying to plug the hole with chains of heavy concrete balls.

But by noon Wednesday they were still repairing equipment and a dyke holding back the sludge, team spokesman Rudi Novrianto told AFP.

The audacious but experimental concrete ball plan aims to slow the toxic mudflow by about 50-70 percent.

Five chains of concrete spheres have been dropped into the steaming mud hole since Saturday. The team aims to drop 374 chains in total, though the figure could rise.

Exploratory drilling in May last year by local gas company PT Lapindo Brantas pierced an underground chamber of hydrogen sulphide, forcing hot mud to the surface.

The sea of mud has inundated hundreds of hectares of land, including villages, factories, rice fields and a key highway. It is also threatening to swamp an important railway, which is to be rerouted.

Experts are unsure how long the crater will spew mud if left unchecked, some suggesting it could be years.

related report
Indonesian "mud volcano" consumes concrete balls
Jakarta (AFP) Feb 27 - A plan to use concrete balls to stem the disastrous ooze from an Indonesian "mud volcano" may be revised after they slid deeper than expected, a spokesman said Tuesday.

The balls slid about one kilometre (half a mile) into the crater, roughly twice the depth anticipated, and many more than planned may be required to staunch the mudflow, said the operation's spokesman, Rudi Novrianto.

"Based on our monitoring of Monday's operations, we may later decide to add to the number of ball chains, but the decision will only be made once the initial target of 374 chains have been dropped into the mud hole," he said.

Each chain comprises four concrete spheres weighing 400-500 kilograms (880-1,100 pounds). Five chains had been dropped into the crater up to Monday.

The audacious but experimental plan aims to slow the toxic mudflow -- which has submerged entire villages in East Java province -- by about 50-70 percent.

Basuki Hadimuljono, the head of the team trying to plug the steaming crater, was quoted by the Koran Tempo newspaper as saying that the number of chains required may rise to 1,000 from the initial estimate of 374.

Several geologists, including Edi Sunardi from the University of Pajajaran, have said the scheme will likely fail.

"We are looking at a plane, and you cannot plug such a plane with concrete balls," Sunardi has said, adding the balls might even resurface.

But Andang Bachtiar, a private sector geologist, said the plan still appeared capable of cutting the volume of mud reaching the surface.

He said it was a complex matter whether more balls than planned would be needed at a lower depth.

"If the section at that depth is actually larger than the section at a depth of 500 metres, then more balls may be needed. Basically, it would depend on a lot of factors," he said.

Ari Setiadi, one of the technical experts working at the crater, told AFP it was not necessarily a bad development that the balls had slid deeper than expected.

"What is important is that they fall below the threshold safety level of 100 metres," he said.

Setiadi said the impact of the operation was being monitored carefully, adding: "So far, so good."

Novrianto said ongoing site repairs and the partial collapse of a dyke holding back the mudflow had delayed the resumption of the operation on Tuesday.

He added that the wind kept changing the direction of toxic fumes rising from the crater. They are sometimes blown towards workers, hindering the effort to slow the flow.

Exploratory drilling in May last year by local gas company PT Lapindo Brantas pierced an underground chamber of hydrogen sulphide, forcing hot mud to the surface in East Java's Sidoarjo.

The sea of mud has inundated hundreds of hectares of land and made 15,000 people homeless. It is also threatening to swamp a key railway, which is to be rerouted away from the danger zone.

Experts are unsure how long the crater will spew mud if left unchecked.

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