More scientists are being airlifted off a research vessel trapped by Antarctic ice but rescue flights will have to halt to make way for an advancing cyclone.

Rescue mission spokeswoman Tanya Hacker told BBC News Online that 48 more of the Russian scientists were being taken off the ice-bound ship on Friday.

Added to the 21 removed on Thursday, that would bring the total rescued to 69.

Although Ms Hacker said the numbers had not been confirmed, the airlifts will likely leave just the 38-member crew still on board the trapped ship, the Magdalena Oldendorff.

The Agulhas was about 330km from the trapped ship

Two South African air force helicopters made at least two flights each on Friday from the rescue ship Agulhas, which is about 330 kilometres (180 nautical miles) from the trapped Magdalena.

"There's virtually no wind at the moment, but there is a cyclone approaching so that will all change," Ms Hacker said, speaking from Cape Town, South Africa.

Cyclones are a normal occurrence in the area for this time of year, and conditions are calm in between storms, she added.

Click here for a map of the area

Temperatures have been down around -20 C where the Agulhas is, and -33 C where the Magdalena is trapped.

Because of the weather, the Agulhas has had to begin moving north, out of helicopter range and away from Antarctic ice that has reached 35 cm thick and is thickening about 1 cm a day.

Icebreaker en route

However, an Argentinean icebreaker is expected to arrive on 7 July, and should be able to carve a path through the ice to the Magdalena, allowing the trapped ship to follow behind on the way out.

The rescue helicopters have also now dropped 1.5 tonnes of food and supplies for those remaining on the Magdalena, Ms Hacker said.

The Magdalena had started rationing its food last week.

Ms Hacker said rescuers hoped it would not be necessary to leave a few essential crew members aboard the ship to last the winter, although it was a possibility.

"We don't want to contemplate that as an option. It is a last resort," Ms Hacker said.

She said she was optimistic the ship would be freed by the middle of July.

The Magdalena had been heading back to Cape Town when a massive ice drift blocked its way on 11 June, forcing it to retreat into a bay and call for help.