A plan to drop concrete balls into the mouth of a "mud volcano" in East Java to stem its flow should go into action next week, Indonesian scientists say.

Hot mud and gas have been spewing out of the ground since May 2006; experts warn the torrent could continue for months, if not years, to come.

But the government-approved scheme could halt the flow within two to three months, the team behind the plan says.

Other geophysicists said it was a "long shot", but "could be worth a try".

Dr Umar Fauzi, who developed the idea with a team at the Bandung Institute of Technology, told the BBC News website the work was due to begin on 7 February, following approval from the government and the team managing the disaster.

Engineers will drop 1,000 1.5m-long metal chains into the mouth of the mud leak. Each chain has four concrete balls suspended from it; two with a 20cm diameter and two with a 40cm diameter.

They will begin slowly, Dr Fauzi explained; perhaps dropping five to 10 chains on the first day, then slowly increasing the number until they insert up to 50 chains per day.

Company blamed

"We aim to lower the chains deep down into the neck of the crater," he said. "This will not plug the volcano, but will force the mud to flow around the chain-balls, decreasing the mud's energy and slowing its flow."

Dr Bagus Nurhandoko, who helped develop the scheme, told Nature magazine: "It will make the mud tired. We're killing the mud softly."

The team is uncertain exactly how long stemming the flow could take.

"We will monitor the reaction of the volcano as we progress," he told BBC News. "How long it will take to stop the flow depends on the reaction, but we think it will take maybe two to three months."

The disaster, which began on 29 May 2006 in the Porong subdistrict of Sidoarjo in Eastern Java, close to Indonesia's second city of Surabaya, is thought to have been triggered by the drilling work of gas prospectors PT Lapindo Brantas.

The event has forced many thousand from their homes.

The Indonesian government has been working to halt the mud with a network of dams and by channelling some of it into the sea, but with little success so far.

The cost of this new scheme is estimated at 3 billion rupiah ($330,800); a government spokesman said PT Lapindo Brantas would pay the cost.

The concrete balls method would cost less than other proposed schemes to halt the mud volcano, Dr Fauzi said.

Brian Simpson, an engineer from Arup Geotechnics, said the plan was a "long shot" and would have to overcome many difficulties.

One of the problems, he said, was by slowing the mud down, you would inevitably create pressure, and this pressure could dislodge the blockage or force open another path.

"However, saying that, when swallow holes or pipes form in dams, it is quite a normal procedure to throw in some fairly coarse material to gradually dam it up," he explained.

What it might do, he added, would be to buy the Indonesian authorities some time to create a more effective and final solution.

"However, now this volcano has been flowing for so long it is going to be extremely difficult to stop, but this scheme is probably worth a try, although I doubt it will work," he said.

Professor Richard Davies, of the Department of Earth Sciences at Durham University, said: "This is completely unchartered territory - nobody has ever done this before. There is a possibility that the pressure may build, forcing open other vents, possibly exacerbating the situation."

Dr Mads Huuse, a geophysicist at the University of Aberdeen, said: "I don't think this idea has ever been tried before.

"If the mud doesn't just whirl straight past these balls, it could work.

"We think this is a man-made volcano caused by the drilling, and it could really go on for a very long while. Already 10,000-11,000 people are homeless," he added.

"It would be wonderful for them if this works."