Jakarta - An Indonesian "mud volcano" that has displaced 15 000 people briefly stopped spewing toxic sludge for the first time in more than nine months, baffling scientists, an official said on Tuesday.

"The flow of mud coming out of the crater suddenly stopped for about 30 minutes shortly before noon (05H00 GMT) on Monday," said Rudi Novrianto, a spokesperson for the government team trying to plug the flow.

"None of our team members knows for sure what happened and we are still trying to determine how it happened," Novrianto said.

The steaming crater, located near Indonesia's second-largest city of Surabaya, merely bubbled during the pause, he said.

The temporary hiatus was the first since the mud hole began spewing sludge in May. Indonesian experts are trying to slow the flow by dropping chains of heavy concrete balls into the funnel, a bold plan some say will not work.

The pause was probably unrelated to the hundreds of chains already dropped into the mud hole, Bagus Endar Bachtiar Nurhandoko, an official from the team battling the crater, told the Kompas newspaper.

He said the brief halt may have occurred because parts of the funnel collapsed, creating a temporary obstruction that was eventually cleared by pressurised gas in the crater.

"We were worried that an explosion would follow, but it turned out not to be the case," Nurhandoko was quoted as saying.

Experts have already dropped an initial target of 374 chains, each comprising four concrete balls, into the crater. They hope to narrow the funnel and obstruct the sludge in a bid to curb the flow by up to 70 percent.

Another 500 chains are to be sunk into the mud hole, but Novrianto said the team was still awaiting a fresh supply of concrete balls.

Some of the experts working to calm the crater have said the chains appear to have slowed the mudflow. They are working to confirm the assessment with official measurements.

The hot mud began bubbling up from deep underground in late May last year after exploratory gas drilling at the site by a local firm, PT Lapindo Brantas.

The sludge has inundated about 600 hectares, including many homes and factories, leaving thousands of people homeless and jobless.

The mud also threatens to swamp a key railway, which is to be rerouted away from the danger zone.