An oil rig which ran aground in Alaska has been refloated and is being towed to a nearby harbor, with no signs of pollution seen, officials said Monday.

Salvage experts were on board oil giant Shell's Kulluk mobile drilling unit when it was pulled back out to sea late Sunday, and was heading for Kiliuda Bay for inspection, they said.

"The Kulluk was refloated and the vessel condition assessed. The tow is now ongoing and the vessels are in Kiliuda Bay approaching the anchorage," said incident commander Sean Churchfield, Shell Alaska's operation manager.

"Once the Kulluk arrives in Kiliuda Bay, a more detailed assessment will take place before any decisions are made regarding the forward plans for the Kulluk," he added at a briefing.

The structure had 150,000 gallons of ultra-low sulfur diesel and roughly 12,000 gallons of oil and hydraulic fluid, according to media reports in the far northwestern US state.

But Alaska state coordinator Steve Russell said: "The crew of the Kulluk reported no evidence of pollution during its removal from the site," adding that an infrared survey of the spot where it was grounded also showed no spill.

The rig was being towed to Seattle when it broke free in heavy seas on New Year's Eve and washed ashore near Kodiak Island, some 300 miles (480 kilometers) southwest of Anchorage.

Environmental group Greenpeace said the grounding highlighted serious problems. "The battered rig may finally be free, but after this latest fiasco Shell's reputation is in tatters," said Greenpeace campaigner Ben Ayliffe.

"The time has come for the US government to act. It is now patently clear that it is impossible to drill for oil safely in the Arctic."

The recovery operation is being led by Smit Salvage, which has assisted in hundreds of operations around the world, including that of the Costa Concordia cruise ship off the coast of Italy last year.

Kodiak Island is a few hundred miles from Prince William Sound, where the "Exxon Valdez" oil tanker spilled around 11 million gallons (40 million litres) in March 1989, in one of the world's worst environmental disasters.