Four climbers from Germany, South Korea, China and Canada have died returning from the summit of Mount Everest, tour agents and officials said Monday, with one other mountaineer also missing.

The 61-year-old German and the South Korean aged 44 died on the south face of the world's highest peak on Sunday, Ang Tshering Sherpa of the Kathmandu-based Asian Trekking adventure agency said.

"We are sad to announce the death of Eberhard Schaaf, of Germany, at the south side of the summit of Mount Everest," Sherpa told AFP.

"The medical staff at the Himalayan Rescue Association believe the cause of death to be altitude sickness."

Sherpa said South Korean Song Won-Bin, who had been missing since Saturday, died at "The Balcony", an area near the top of the 8,848-metre (29,029-feet) peak.

The Seoul-based Yonhap news agency said Song had collapsed due to altitude sickness and fallen off a cliff, quoting a diplomat at the South Korean embassy in Kathmandu.

It said the climbers were part of a team of old classmates from the same high school in the central city of Daejeon.

About a dozen members flew to Nepal at the end of March to mark their school's 50th anniversary by climbing the peak. They were due to return home later this month.

Tilak Pandey, a tourism ministry official at Everest base camp, told AFP separately that a 33-year-old Nepali-born Canadian woman named Shriya Shah had also been killed on Sunday.

Shah was born in Kathmandu and grew up in Mumbai, India, according to her website. She lived in Toronto and described herself as "an entrepreneur, political activist, social worker, and above all, a daring lady".

Speaking of her Everest expedition, she wrote: "This is my dream and passion, and want to do something for my country. Nothing is impossible in this world."

Another ministry official, Deependra Paudel, told AFP 55-year-old Chinese climber Ha Wenyi had also been found dead, at 8,600 metres.

"Most of these deaths occur due to high altitude sickness," said Sherpa, adding that a Nepali mountain guide was also missing.

"Climbers spend their energy on the ascent and they are exhausted and fatigued on the descent."

Mountain guides from Sherpa's company told him of the discovery of the body of another climber on the north side of Everest but no further details were available to verify the death.

Everest's "death zone", the region above 8,000 metres, earned its name because it is almost impossible to survive the biting temperatures and lack of oxygen there for more than 48 hours.

Conditions have been particularly hazardous this year, said Nepalese government official Gyanendra Shrestha, with high winds and heavy snowfall delaying the construction of makeshift bridges over precipices.

"The first expedition reached the top only on May 18 whereas last year it was on May 5," he said.

"With so many people trying to reach the top there was a traffic jam. The next forecast for good weather is between May 24 and 26. By May 28, the ice will start melting and expeditions will have to be called off."

Two Nepali Sherpa climbers died on Everest in April, one falling into a crevasse at 5,900 metres and the other succumbing to altitude sickness at base camp.

Nearly 4,000 people have climbed Mount Everest since 1953 when Sir Edmund Hillary of New Zealand and Sherpa Tenzing Norgay first scaled it.

More than 200 people have died on the slopes of the giant peak.