The company responsible for an oil spill off the West Australian coast has acquired access to more Australian oilfields.

PTTEP Australasia has so far been unable to plug the leaking Montara rig in the Timor Sea despite numerous attempts over nine weeks.

The company was due to make its fourth attempt to plug the leak yesterday, but that was deferred.

Despite the company's failure to stop the leak, it yesterday gained control of two more offshore oil fields and five exploration licences.

The move was supported the Foreign Investment Review Board and the Federal Resources Minister Martin Ferguson.

A spokesman for Mr Ferguson defended the decision saying there was nothing to suggest the company had done anything untoward.

PTTEP Australasia says it will use a new specialised tool in its next attempt to plug the leak, which may take place this weekend.

An environmental group said on Friday that the oil spill is worse than it feared.

The World Wide Fund for Nature completed a three-day boat trip of the area and said the spill is having a significant impact on marine life.