Blaze breaks out as workers attempt to plug leak with heavy mud to stop oil slick which is threatening marine wildlife

A massive fire has broken out on an Austalian oil rig and delayed plans to plug the leak that has been spilling oil into the Timor Sea since August.

The blaze started on Sunday when workers began pumping heavy mud into a leaking well casing. An estimated 400 barrels of oil a day have escaped from the hole since 21 August, threatening marine wildlife over an area ten times the size of London.

Australia's government today promised an investigation, the latest drama in a 10-week saga to plug the hole.

PTTEP Australasia, which operates the rig, said no one was injured and nonessential workers were evacuated after the fire broke out on the West Atlas rig and Montara wellhead platform.

Officials had planned to pour more mud into the leak on Monday in the hopes of removing the source of fuel from the fire, which was sending massive plumes of smoke into the sky. But the company said it was mixing 4,000 barrels of heavy mud, and would not be ready to pour it down the well until Tuesday.

On Sunday, PTTEP Australasia chief financial officer Jose Martins said the company doesn't know how the blaze started.

"Presently there are many unanswered questions, including what caused the fire," Martins told reporters in Perth. "Our sole focus now is the safety of all personnel, bringing the fire under control and completing the well kill."

Federal resources minister Martin Ferguson said that once the spill is contained he would launch an official inquiry. "Our requirement is to assess the cause of the accident and any lessons to be learnt, and that could lead to a change in the regulatory environment," he told Australian Broadcasting Corporation radio.

Ferguson later told reporters in Melbourne that if PTTEP was "found to have been at fault with respect to any of their responsibilities, then any potential action will be appropriately considered at the time."

The oil slick from the rig, about 150 miles off Australia's north-west coast, now stretches across thousands of milesof remote ocean. Indonesia said last week that thousands of dead fish and clumps of oil have been found drifting near its coastline.

Prime minister Kevin Rudd said today he was "deeply disturbed" at the latest turn of events on the rig, signalling the government's rising frustration that fixing the spill is taking so long.

"Do I think this is acceptable? No, I don't," Rudd told Fairfax Radio Network. "Are we angry with this company? Yes we are. Are were trying to do everything we can to get this under control? You betcha."