A Shell oil drilling rig grounded off the coast of an Alaskan island doesn't appear to be leaking any oil, officials say.

The Kulluk oil rig broke free of its mooring while being towed late last week, and after multiple attempts to retrieve it, it eventually ran aground near the eastern shore of Kodiak Island in the North Pacific.

Eighteen crew members were on the rig, but all were rescued.

Environmentalists are concerned that the rig smashing unencumbered on jagged rocks in the area would release oil or other toxins into the water, but that doesn't appear to be happening, the U.S. Coast Guard's on-scene response co-ordinator, Capt. Paul Mehler, said Tuesday.

"There is no sign of a release of any product," Mehler told a news conference, adding the rig appears to be stable.

The rig is carrying 541,300 litres of diesel and roughly 45,425 litres of lube oil and hydraulic fluid. It was built with a double-sided hull of reinforced steel that is almost eight centimetres thick.

The rig recently underwent $292 million US in improvements before being briefly put into service looking for oil near the Arctic last summer and fall.

A coast guard plane and a helicopter flew over the Kulluk on Tuesday, but high seas and strong winds prevented crews from boarding the ship to check for any damage.

Endangered species in area of rig

A team of coast guard, local government and Shell officials was mobilizing spill response equipment and preparing a plan in the event of a spill.

The area is home to at least two endangered species, as well as harbour seals, salmon and sea lions.

Although the impact is minimal for now, the incident is underscoring the concerns of those opposed to drilling in the region, and who worry a blowout would have a disastrous impact on the sensitive arctic ecosystem.

Congressman Ed Markey, the top Democrat on the House's natural resources committee, expressed his concerns in a statement Tuesday.

"Oil companies keep saying they can conquer the Arctic, but the Arctic keeps disagreeing with the oil companies," Markey said. "Drilling expansion could prove disastrous for this sensitive environment."