Image caption Officials say all on board the stranded ship are safe and well

Sea ice conditions in the Antarctic are likely to delay plans to take passengers off an ice-bound ship, officials say.

Rescue co-ordinators based in Australia said it is now likely the airlift will not go ahead on Thursday as hoped.

They had earlier been optimistic that helicopter operations would begin soon as weather conditions had improved.

The ship, the Akademik Shokalskiy, with 74 passengers and and crew aboard, has been stuck since Christmas Eve.

Earlier efforts by three different icebreakers to free it failed.

The planned rescue on Thursday was to have ferried groups of 12 passengers by helicopter to the Chinese icebreaker Xue Long, from where they would be taken by sea to the Australian icebreaker Aurora Australis.

Alternative measures

"Sea ice conditions are now likely to delay today's planned rescue of passengers from the MV Akademik Shokalskiy," said a statement from the Australian Maritime Safety Authority's Rescue Coordination Centre (RCC).

"It is now likely the rescue will not go ahead today."

The current sea ice conditions "prevent the barge from Aurora Australis from reaching the Chinese vessel".

The barge is needed because weight restrictions mean the Chinese helicopter cannot land on the Aurora Australis. Landing next to Aurora Australis is "not safe" at the moment, the RCC said.

While the preferred and safest option was to rescue the passengers in a single operation, "alternative measures" to complete the rescue operation are now being explored.

RCC has been told that all 52 passengers - comprising scientists and tourists - will leave the Shokalskiy. All 22 crew are expected to remain on board.

The helicopter operations alone are expected to take five hours.

Image caption The research ship has been stuck for more than a week

The Return to Mawson's Antarctica The 2013 Australasian Antarctic Expedition repeats scientific investigations made by Douglas Mawson and his team between 1911 and 1914

Andrew Luck-Baker and Alok Jha are reporting on the expedition for Discovery on the BBC World Service

Listen to Part One, Part Two and Part Three of their story, or get the Discovery podcast

The Shokalskiy was trapped on Christmas Eve by thick sheets of ice driven by strong winds, about 1,500 nautical miles south of Hobart - the capital of the Australian state of Tasmania.

Despite being trapped, the scientists have continued their experiments, measuring temperature and salinity through cracks in the surrounding ice.

The vessel is being used by the Australasian Antarctic Expedition to follow the route explorer Douglas Mawson travelled a century ago.