SYDNEY (AFP) - Sea ice delayed a planned helicopter evacuation of a Russian research vessel trapped in Antarctica, with Australian maritime officials saying the rescue was unlikely to go ahead on Thursday as earlier planned.

The Australian Maritime Safety Authority (AMSA) said the operation involving the airlifting of 52 passengers from the Akademik Shokalskiy, which had been set to involve a Chinese helicopter, no longer looked possible due to the adverse conditions.

The ice was preventing a smaller vessel from the Australian icebreaker Aurora Australis from reaching Chinese icebreaker the Xue Long, which is nearer to the stranded ship.

The plan was to fly passengers to the Chinese vessel before they were transferred to the Aurora Australis by boat.

Leaving passengers on the Xue Long is not a workable option and AMSA said it is "now likely the rescue will not go ahead today".

"The preferred and safest option at this stage is to ultimately transfer the passengers on to Aurora Australis," AMSA said.

"The preferred option is to wait for conditions that will allow the rescue to be completed in a single operation to reduce unnecessary risk."

AMSA said "alternative measures to complete the rescue operation are now being investigated".

The Akademik Shokalskiy, carrying 52 passengers and 22 crew, has been trapped in pack ice 100 nautical miles east of the French base Dumont d'Urville since December 24.

Several icebreaking attempts have failed to reach the ship, forcing a more complex helicopter rescue.