Jakarta – A danger zone declared around an Indonesian "mud volcano" spewing vast amounts of toxic sludge, which has displaced 15,000 people, may have to be widened, an expert said Wednesday.

A taskforce declared a 440-hectare (1,087-acre) area – including four villages submerged by the mud – uninhabitable after the crater began oozing in May in Sidoarjo district, East Java.

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.