An Antarctic rescue mission has suffered a setback after a Chinese icebreaker had to abandon its bid to free a ship stranded in sea ice.
A group of scientists, explorers and tourists has been stuck on the Russian ship MV Akademik Shokalskiy since Christmas Day.
The ship, with 74 people on board including four New Zealanders, sent a distress call after becoming trapped in heavy sea ice, which has continued to expand around it over the past three days near Antarctica.
Three ice-breaking ships were sent to free the stranded vessel, located more than 1500 nautical miles south of Hobart.
China's Snow Dragon icebreaker was the first to arrive and rammed through much of the sea ice until it came within six-and-a-half nautical miles of the stranded vessel.
But in a setback to the rescue mission, the icebreaker had to turn back on Saturday after it, too, encountered heavy sea ice that threatened the ship's own safety.
"It can't break through any further," said Australian Maritime Safety Authority (AMSA) spokeswoman Andrea Hayward-Maher.
"That vessel is now making its way back from the direction it came in." A French icebreaker, L'Astrolabe, and Australia's Aurora Australis are still heading to the remote area, but the Aurora was not expected to arrive until Sunday.
The stranded ship had been undertaking the Spirit of Mawson voyage, which is retracing Sir Douglas Mawson's Antarctic expedition.
The privately funded expedition, led by Professor Chris Turney of the University of NSW, was to return to the huts used by Mawson a century ago. The huts had been inaccessible for many years because of an iceberg.
They plan to continue their expedition after they are freed, Turney said.
Despite the interruption to the expedition, the scientists have continued their research while stuck, counting birds in the area and drilling through the ice surrounding the ship to photograph sea life.
Hayward-Maher said authorities are still weighing up their options on how to launch the second rescue bid.
The Chinese ship was rated at being able to break through ice up to a metre deep.
The Aurora is rated at a slightly-higher capability, she said.
But if none of the icebreakers can reach the ship, a second option may be to consider ferrying trapped explorers with a helicopter onboard the Chinese vessel.