A six-member salvage team was lowered Wednesday onto an oil rig that ran aground in Alaska to assess options as the US Coast Guard vowed to prevent any ecological threat, officials said.

The team spent three hours on board the oil giant Shell's Kulluk mobile drilling unit after an improvement in weather conditions, which had prevented any such operation since the rig's grounding on New Year's Eve.

"Calmer conditions this morning created a window that enabled the assessment to take place," said an updated statement from a Unified Command including the US Coast Guard and Shell.

As well as the experts, a Coast Guard helicopter "also delivered a state-owned emergency towing system to the Kulluk, which will be used during salvage operations, it added.

"The information gained from the on-site assessment will be invaluable in helping to evaluate the available options for freeing the rig from its grounded position," it said.

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 km) southwest of Anchorage.

It has some 150,000 gallons of ultra-low sulfur diesel and roughly 12,000 gallons of oil and hydraulic fluid, according to the far northwestern US state's KTUU television station.

"The Conical Drill Unit (CDU) Kulluk ... remains grounded but stable," said the incident command managers, who organized flights earlier Wednesday to assess the situation from the air.

Eighteen crew members had already been evacuated from the rig before it broke free late Monday, KTUU reported. By Tuesday it was described as "upright and rocking with a slow, but stable motion."

Shell said three people suffered minor injuries during the response to the Kulluk's grounding, according to the television station.

The Federal Aviation Administration (FAA) has set up a no-fly zone around the rig, and the Coast Guard is maintaining a safety zone of one nautical mile, said the latest update.

Alaska's Coast Guard chief, Rear Admiral Thomas Ostebo, said his primary concern was ensuring the unified command has the personnel, assets and equipment needed to safely salvage the Kulluk.

"I have overflown the Kulluk twice and am fully aware of the remote location, weather challenges and the extensive plans that will have to be developed to ensure this incident is managed in safe and effective manner," he said.

"This is a very large and complex response and it is important that the American public and our elected officials understand the dangerous and difficult challenges being faced by the response crews."

He added: "We are continuing our collaborated response with other shareholders in the unified command until the grounded Kulluk no longer poses a threat to the pristine Alaska maritime environment."

The 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.