Two more climbers have died on Mount Everest, taking to six the death toll on the world's highest mountain in the past month, fuelling safety concerns among mountaineers, officials said on Monday.

A 54-year-old Australian, Francesco Enrico Marchetti, died on the Tibetan side of Everest, while Vladimir Strba, 48, of Slovakia, perished near a feature called the Balcony in the south in Nepal, officials said. Both climbers died on Sunday.

Marchetti died of apparent altitude sickness about 8,300 metres (27,230 feet) above sea level on his summit approach, said Navin Trital of the Expedition Himalaya company that coordinated logistics for the climber.

"He fell sick and died while being brought down to a lower camp," Trital told Reuters in the capital, Kathmandu. He said he was unable to give details because of poor communications with the team.

Strba died at around 8,400 metres (27,500 feet) on the Nepali side in the "death zone", where the air is very thin, Tourism Department official Gyanendra Shrestha said.

He also said the exact circumstances leading to Strba's death were not clear.

An American climber died on Sunday, while rescuers spotted the body of an Indian climber, who fell 200 metres (650 feet) to his death after scaling the peak a day earlier, a Tourism Department official said.

On April 30, famed Swiss climber Ueli Steck fell to his death near Mount Everest while preparing to climb it. An 85-year-old Nepali man died at base camp this month while trying to set a record for the oldest climber.

This year's Everest expeditions have been confronted by bad weather and high winds.

"There have been deaths on both sides. While few details are provided, it appears these were associated with altitude, not weather," American blogger and climber Alan Arnette said in a post.

Arnette reckons that an annual average of 6.9 climbers has died since 2000 trying to scale the world's tallest peak, meaning that fatalities so far are in line with the typical annual figure.

Nepal has cleared 371 climbers to Mount Everest in the current season, which ends this month. Nearly 200 climbers went to Tibet, from where the 8,850-metre (29,035-foot) Everest summit can also be scaled.