SYDNEY - AIRCRAFT sprayed chemicals to break up a large oil slick off Australia's northwestern coast on Sunday as environmentalists expressed fears for rare wildlife from oil gushing into the sea from an uncapped well.

The Australian Maritime Safety Authority (AMSA) said the slick from the West Atlas offshore drilling rig had lengthened overnight from an estimated 8 nautical miles (15 km) on Saturday.

The first sortie of chemical dispersant sprayed on Sunday from a C-130 Hercules aircraft appeared to have started to break up the slick, a spokesman said. However, the clean-up cannot be completed until the well is capped, which experts say may take days.

'The indications are that the application of the dispersant has been successful,' said AMSA spokesman Tracey Jiggins.

'We are certainly prepared for an ongoing operation.' Rig operator PTTEP Australasia, a unit of Thailand's PTT Exploration and Production PCL , has flown in a team of experts to try and determine how to cap the well, which first began to leak oil and gas on Friday.

An air exclusion zone had been set up and ships have been advised to stay more than 20 nautical miles away from the rig, which is currently considered too dangerous to board.

Environmental group WWF called on Sunday for changes to preparations for such disasters, pointing out it took three days for the first dispersant to be sprayed, although the region is considered a critical area for biodiversity.

'From a global scale this is one of the most important places on the planet for ocean wildlife,' Ms Gilly Llewelyn, WWF Australia's director of conservation, told Reuters.

Among the animals affected were three endangered species of turtles, plus sea snakes, she said. Even a pygmy blue whale has been monitored there in what seemed to be an 'oceanic highway' linking the Indian and Pacific Oceans.

Many of those animals breathe air and periodically need to surface, and could surface in the middle of the slick, she said. -- REUTERS