Sydney  - A Thai oil exploration company said Tuesday its second attempt to plug a leaking Timor Sea oil well had failed.

The West Atlas drilling platform operated by PTTEP Australasia in the Montara field 690 kilometres west of Darwin has been leaking around 400 barrels of oil and gas a day since August 21.

PTTEP brought in a mobile rig from Indonesia and is drilling 2.6 kilometres into the seabed to try and intersect the leak and plug it with drilling mud. A first attempt failed last week.

"The closer we get to the target with each pass the more certain we become of its location," PTTEP Australasia director Jose Martins said in a statement.

Martins, the company's chief financial officer, said another attempt would be made at the end of the week.

Conservation organization WWF has reported that the oil slick could cover up to 15,000 square kilometres. There is a 40-kilometre exclusion zone and so the WWF only has rough estimates of the scale of the pollution.

The Australian Maritime Safety Authority (AMSA) has been using C-130 Hercules aircraft flying from Darwin each day to drop dispersant over the slick.

AMSA spokesman Mick Spinks said most of the oil had stayed within the vicinity of the rig and had begun to emulsify and congeal.

"It's clumped itself tighter as part of the weathering process and has formed a mousse-type of substance that will eventually break down," he said. (dpa)