Rescuers were planning an aerial operation on Sunday to spot several climbers missing on K2, the world's second highest mountain, as a Swedish survivor said 11 mountaineers may have died in the tragedy that hit international expeditions two days ago.

Eight expeditions were affected on Friday when a big chunk of ice fell from the mountain, located in the Pakistani Karakoram segment of the Himalayan range, at a height of 8,211 metres, taking a large number of fixed ropes with it.

Some 20 to 25 climbers were heading towards the 8,611-metre summit or descending after reaching the top.

At least nine climbers - two Nepalese, three Koreans, one each from Norway, Serbia, Ireland and Pakistan - were confirmed dead by retired Brigadier Mohammed Akram, spokesman for the Alpine Club of Pakistan. On their way back, both slipped and fell into a crevasse.

"The body of Serbian climber Dren Mandic was found near camp three and was buried by fellow climbers," a spokesman for Nazir Sabir Expedition said.

But a Swedish survivor feared in a radio interview that up to 11 climbers may have died.

The accident was due to "bad knowledge about the mountain, people that did not turn around in time and relied on others rather than their own skills," the man surnamed Strang, 31, told Swedish radio news from a base camp at 5,100 metres after making his descent from some 8,000 metres.

Officially, however, half a dozen climbers, including two Austrians, and one each from from France and the Netherlands have been declared missing.

A Dutch Norit K2 Expedition said its three other stranded members Wilco van Rooijen, Cas van de Gevel and Pemba Sherpa managed to reach camp two at less than 7,000 metres on Sunday afternoon.

"The team members will rest and drink a lot," a statement posted at the expedition website said. Two of them were suffering from frostbite.

The group said they were preparing for aerial operations to search the stranded climbers and move the injured to the hospitals.

"K2 base camp is organizing a fly-over by plane above the flanks of K2 to locate any missing climbers. The plane is on standby, but we have to wait for the clouds to clear," the group said in their internet posting.

Helicopter transportation for the injured was scheduled for Monday morning.

Strang, the Swedish climber, told the online edition of Stockholm daily Aftonbladet that members of his expedition decided Friday to abort their effort to reach the peak but heard over the radio that other teams were in trouble higher up.

He and others continued up to Bottleneck Couloir to try to ferry down the body of a dead climber Strang knew well.

When they arrived there, several climbers were exhausted and dehydrated, several had severe frostbite, Strang said.

He then began the descent with the body of his friend on his back.

On the way down, a guide from Pakistan lost his footing and almost pulled Strang with him. The guide plunged to his death, Strang said.

In a condolence message to the family of climber Gerard McDonnell, Irish President Mary McAleese said, "Following so closely on their righteous pride, and that of the country, at Gerard becoming the first Irish person to scale K2, it is truly heartbreaking that they must now contemplate the loss of a beloved son and brother."

McDonnell became the first Irish person to reach the summit of K2 on Friday. He was feted as a hero earlier in the year after completing the first Irish expedition to the South Pole.