WELLINGTON (AFP) - A United States ship was on Saturday towing an Australian fishing trawler to open water after freeing it from Antarctic pack ice, rescue authorities said.

The 63m Antarctic Chieftain became trapped in ice some 1,650km north-east of McMurdo Sound on Tuesday, damaging its propeller. The US Coast Guard cutter Polar Star was ordered to cross through 720km of iceberg-strewn waters to the ship's aid.

The New Zealand Rescue Coordination Centre, which oversees the Southern Ocean search and rescue zone, said the US ship, which has a reinforced icebreaker hull, reached the trawler early on Saturday after breaking through the ice around it.

"The crew on the Polar Star then rigged up tow lines and began to tow the Antarctic Chieftain to open water," rescue mission coordinator Conrad Reynecke said in a statement. "They are making slow but steady progress and are currently approximately 60 nautical miles from clear water."

The Americans had earlier deployed a remote controlled mini-submarine to assess the damage to the Antarctic Chieftain's propeller and gauge if it could travel under its own steam.

"The (propeller) blades were assessed as too badly damaged for the vessel to be able to use them for propulsion from the ice field," Mr Reynecke said.

Weather conditions continue to be favourable but Polar Star commanding officer, Captain Matthew Walker, said earlier that the ice conditions were "much more formidable than expected".

Capt Walker earlier said his crew were breaking the ice around the vessel after their arduous journey, which included dodging large icebergs. "We are on scene and progressing well with the rescue," he said.

New Zealand rescue coordinators say the trawler's 26 crew members are not at risk and there has been no oil leak in the environmentally sensitive area.

A New Zealand-flagged trawler Janas is also travelling to the area to provide assistance if required but is not expected to reach the vessels until late on Monday.

The Antarctic Chieftain, built in 2002, is licenced to trawl for Patagonian toothfish, a slow-growing species that has a type of anti-freeze in its blood to deal with the punishing southern conditions.

The ship's owners, Australian Longline, said on their website that the vessel spends six months at a time in Antarctic waters fishing the prized species, which is also known as "white gold" for the profits it can yield.