An icebreaker has set off from Argentina to rescue a research ship trapped in ice off Antarctica with 107 people on board.

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The Almirante Irizar will join a South African ship on the mission to rescue the Magdalena Oldendorff, a German vessel chartered by Russia, which has been frozen in a bay on the Princess Astrid Coast since 11 June.

It's a dangerous time of year in Antarctica because of the winter and the difficulty with the ice floes

Argentine captain Raul Benmuyal

Rescuers say time is critical as poor weather and darkness are closing in for the 79 scientists and 28 crew members aboard the Magdalena Oldendorff.

"We're very, very happy that the Almirante left today," Tanya Hacker, spokeswoman for the South African rescue mission, told BBC News Online.

"The entire operation hinges on weather conditions on a day-to-day basis."

The South African rescue ship Agulhas, which left Cape Town 10 days ago, has already been delayed by high winds and temperature drops.

Last week the stranded scientists and crew began rationing their remaining food.

Helicopter shuttle

"It's a dangerous time of year in Antarctica because of the winter and the difficulty with the ice floes," the icebreaker's captain, Naval Commander Raul Benmuyal, warned.

The ship was heading for home when it got stuck in ice

The Argentine ship aims to break a path through the ice to the trapped vessel, allowing the Magdalena Oldendorff to make its way out under its own steam.

Commander Benmuyal said the priorities were getting the German ship out to sea and restocking it with fuel and supplies.

Only if attempts to free the ship fail will personnel be evacuated.

"If we see we're not able to get the Magdalena out we will set up a shuttle service to fly non-essential staff and crew off the Magdalena," Ms Hacker said.

The Almirante Irizar has two Sea King helicopters on board, while the Agulhas is carrying two Oryx helicopters.

The Almirante is expected to take about two weeks to cover the 8,000 kilometres (5,000 miles) from Buenos Aires to the stranded vessel.

'Everything OK'

Meanwhile, the Agulhas, which has already reached the ice fields about 690km (430 miles) from the Magdalena, is pressing on as far as it can safely go.

Already the sea water is 90% ice, but it is still soft enough for the ship's ice-strengthened hull to pass through.

Once the Agulhas cannot push any further, it will start launching reconnaissance helicopter flights while awaiting the arrival of the icebreaker, Ms Hacker said.

Food supplies on the Magdalena Oldendorff are expected to last until 15 July.

Ms Hacker said an email received from the Magdalena's captain on Tuesday had said: "Everything on board is still OK".

She said that the ship remained in exactly the same position and that there were sufficient rations although the crew had complained of a shortage of items such as cigarettes, sugar and butter.

The Magdalena Oldendorff had been heading back to Cape Town before a massive ice drift blocked its way and forced it to retreat and call for help.

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