Image caption The Polar Star is the US Coast Guard's only active heavy polar ice-breaker

A US ice-breaker is to help two ships that are stuck in thick ice in Antarctica following a rescue mission.

The Polar Star has been asked to cut a path through the ice in order to free the Xue Long and the Akademik Shokalskiy, the US Coast Guard said.

The Chinese boat, Xue Long, became ice-bound after helping to rescue 52 passengers stranded on Akademik Shokalskiy on Thursday.

The Russian research vessel has been trapped since 24 December.

Safety of life

The US ice-breaker is responding to requests for assistance from the Australian authorities as well as from the Russian and Chinese governments, the US Coast Guard said in a statement.

"Our highest priority is safety of life at sea, which is why we are assisting in breaking a navigational path for both of these vessels," said Vice Adm Paul Zukunft, commander of Coast Guard Pacific Area.

Image caption The Xue Long helped to rescue passengers trapped on the Akademik Shokalskiy since 24 December

Image caption A helicopter from the Chinese vessel transported the passengers to a ice floe next to the Aurora Australis

Image caption The Xue Long has confirmed on Friday that it too had become stuck in the ice

The Polar Star, which is specifically designed for open-water icebreaking, was due to leave Sydney, Australia, on Sunday and is expected to arrive at the stranded ships in a week's time, according to the Australian Maritime Safety Authority (Amsa).

It comes after the Xue Long confirmed it was trapped following a rescue mission to remove passengers on board the Akademik Shokalskiy.

On Thursday, a helicopter from the Chinese ship transferred 52 people from the Akademik Shokalskiy to the Australian vessel, the Aurora Australis.

Most of the Russian crew-members stayed on board the Akademik Shokalskiy.

The Russian research vessel became trapped by thick floes of ice driven by strong winds, about 1,500 nautical miles south of Hobart - the capital of the Australian state of Tasmania.

It was being used by the Australasian Antarctic Expedition (AAE) 2013 to follow the route explorer Douglas Mawson travelled a century ago.

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

Several attempts to break through to the ship by sea - by the Xue Long, Aurora Australis and French-flagged L'Astrolabe - failed because of the thickness of the ice.

The team on board said the ship was stocked with food and was in no danger.

There are 111 individuals on the Xue Long and 22 remaining crew on the Shokalskiy.

The Aurora Australis will now transport the rescued passengers to Hobart, once it has completed a re-supply mission at the Casey research base on the Windmill Islands, just outside the Antarctic Circle.

Meanwhile Prof Chris Turney, co-leader of the AAE 2013, has defended the scientific value of the expedition and rejected claims it was a "tourist trip" hampered by poor preparation.

Writing in the UK's Observer newspaper, he said the trip had been struck by bad luck as opposed to human error. He said it was an important scientific expedition and its success would ultimately be measured by peer-reviewed studies.

The Polar Star is 399ft (120m) long with a maximum speed of 18 knots, the US Coast Guard said. It can continuously break 6ft (1.8m) of ice at three knots, and can break 21ft (6.4m) of ice by backing and ramming.

The vessel has been diverted from its mission to break a channel through the ice to resupply the US Antarctic research station on Ross Island - but will return to that task once it has freed the trapped ships.