Scores of Nepalese guides and foreign climbers on Mount Everest packed up Thursday, loading supplies onto yaks and booking helicopters, with the climbing season increasingly in doubt after an avalanche killed 16 people last week.

Expeditions prepared to leave shortly after crisis talks ended with the Nepal government at Everest base camp in the wake of last Friday's worst ever accident on the world's highest peak.

With sherpa guides threatening to boycott the season, officials promised several hundred of them and foreign mountaineers during the talks that their climbing permits would be extended for five years, in a bid to lure the frustrated climbers back to the country in future.

Both sherpas and foreign mountaineers normally have to buy annual permits to scale Everest, with those for a foreign climber costing at least $11,000.

Satisfied with the extension, sherpas said they would now honour their dead colleagues by leaving the mountain, while some climbers said it was too unsafe to scale the 8,848-metre (29,029-foot) high peak this season.

"I see most expeditions packing up now. They are arranging for yaks to carry their supplies," local police official Kumar Timilsina told AFP from base camp.

Guide Nima Tenzing Sherpa told AFP that "most teams are discussing cancellations... several clients have left today in helicopters".

"We are very glad that the government has agreed to extend the validity of the permit to five years," said Tashi Sherpa, who attended the meeting.

"Most of us don't want to climb this year, we have lost so many good people and we want to honour them," Sherpa told AFP.

In an online post Thursday afternoon, South African climber Saray Khumalo announced: "They say the mountain is open but it certainly is not safe for any of us to attempt it this year."

The government has been under pressure to resolve the crisis, amid growing tensions on the mountain and climbing companies cancelling expeditions, citing safety concerns and also fears of violence.

Under fire over its handling of the disaster, the government is desperate to avoid a shutdown of the season that could lead to messy refund claims and a huge loss of revenue for the impoverished country.

The tourism minister insisted the mountain was still open for business, while also reassuring sherpas and climbers about permit extensions, an official at the talks told AFP.

"The minister has said that foreign mountaineers who want to climb this year can go ahead," said Dambar Parajuli, president of the Expedition Operators Association.

-'Time to go home'-

The talks came hours after three more major mountaineering companies announced they had abandoned plans to climb this year.

Leading US company International Mountain Guides said the main route through the Khumbu Icefall, where the avalanche struck a team of guides carrying equipment for their clients, was too dangerous.

"The icefall route is currently unsafe for climbing without repairs by the icefall doctors (highly-skilled guides who fix ropes and repair ladders up the mountain), who will not be able to resume their work this season," IMG said in a statement.

Announcing it was also cancelling its expedition, US company RMI Expeditions said "the risks outweigh the possibility of success".

Peak Freaks, led by Canadian mountaineer Tim Rippel, said "the route in my professional opinion is NOT safe", adding there was a real threat of further avalanches.

Describing a tense environment at base camp, Rippel said sherpas keen to shut the climbing season were threatening violence against those who wanted to continue. Mountaineering companies and officials were also pressuring guides to stay on the mountain, he said.

Three other expeditions have already cancelled their plans to summit.

Sherpas on Tuesday threatened to abandon the season after issuing demands to the government, including higher compensation for the dead and injured, increased insurance payments and a welfare fund.

The avalanche underlined the risks borne by sherpas who carve out the routes and carry gear up the mountain for their foreign clients.

The government offered to set up a relief fund for injured guides using up to five percent of fees paid by climbers, while increasing life insurance payments by 50 percent. Both amounts fall short of the sherpas' demands.

The government, which has earned $3.6 million this year from Everest climbing fees alone, has issued permits to 734 people this season.