Special army and government rescue personnel have intensified their search for four South Korean trekkers and their three Nepali guides who went missing after an avalanche swept a popular trekking route in Nepal's mountains.

All other trekkers who were in the area where the avalanche swept the Mount Annapurna trekking trail about 150km (90 miles) northwest of the capital Kathmandu on Friday have been safely rescued and flown to safer areas, Department of Tourism official Mira Acharya said.

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The avalanche hit part of the Mount Annapurna circuit trekking route after heavy snowfall earlier in the week.

By Friday, snow was neck-deep and several avalanches had occurred, a veteran climber who was rescued by helicopter said.

"We had crossed the area just three hours before the big avalanche hit the area, which has a few rest stops and lodges," Phurba Ongel Sherpa told The Associated Press news agency on Monday.

On the way back, he said he saw the area from the helicopter where the avalanche had hit and it was blanketed with snow.

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"There is no way that anyone buried in that pile could have survived," said Sherpa, who has conquered Mount Everest nine times.

More avalanches, heavy snow and poor visibility hampered the search on Sunday, officials said.

Relatives of the missing Koreans arrived in Kathmandu alongside several officials sent by Seoul to help with the emergency rescue efforts, Ang Dorjee Sherpa of the Korean Alpine Federation told AFP news agency on Sunday.

The missing group was near the Annapurna base camp around 3,230 metres (10,600 feet) above sea level when the avalanche struck.

In this photo taken on Saturday, trekkers are rescued by a helicopter a day after the avalanche hit Mount Annapurna trail [Phurba Ongel Sherpa/AP]

Helicopters and drones

Rescuers were working with Korean officials to deploy drones in the search on Monday, said Dilip Gurung of the tourism management committee in Chhomrong, which lies on the trekking route.

"It is difficult for people to go. We will try to fly drones to help find something," Gurung said.

On Saturday, four rescue helicopters evacuated 200 people, including 140 foreigner hikers, stranded around Annapurna and other nearby mountains after the incident, police officials said.

Tek Gurung, a guesthouse owner aiding the search operation, said more than two metres of snow (6.6 feet) had fallen on the trekking trails and it was "extremely difficult" to search the snow-covered area on foot.

Six of the missing were part of the same expedition, while one Nepali porter was escorting a different group.

The four foreigners - two men and two women - were part of an 11-member team of South Korean nationals. Others have safely descended.

Education officials in Seoul said they were part of a team of volunteer teachers working with children in Nepal.

Mount Annapurna is the world's tenth-highest mountain at 8,091 metres (26,545 feet). Its base camp is known for its spectacular beauty and thousands of trekkers go there every year.

The incident comes as the annual trekking season in Nepal, home to eight of the world’s 14 highest mountains including Mount Everest, is drawing to a close.