Itanagar, India: Helicopter teams began evacuating critically injured climbers at Mount Everest's base camp on Sunday morning, but the effort came to an abrupt halt when a significant aftershock triggered more avalanches and fears of additional casualties at the world's highest peak.

Dozens of climbers and sherpas, their Nepali guides, remain trapped on the side of the mountain at two camps that sit above where the avalanche fell, climbers said in tweets and posts on social media.

Ropes and other equipment left in place to help them descend had been swept away in Saturday's avalanche.

Daniel Mazur, a climber trapped at camp 1, tweeted on Sunday: "Aftershock @ 1 pm! Horrible here in camp 1 Avalanches on 3 sides. C1 [camp 1] a tiny island. We worry about the icefall team below. Alive?"

Colonel Rohan Anand, a spokesman for the Indian Army, which had a mountaineering team training on Everest at the time of the disaster, said the rescue effort also had been hampered by communications difficulties and weather.

The magnitude 6.7 aftershock occurred about 1pm Nepal time, the US Geological Survey said.

The army said that 19 people had died at the Everest base camp on Saturday after an enormous sweep of ice, rocks and snow tumbled towards the camp in an avalanche triggered by Nepal's deadly earthquake, which killed about 2500 people.

The army had rescued 61 climbers, mostly foreign tourists.

The wind that accompanied the avalanche "completely pulverised and blew the camp away," American climber Jon Kedrowski, who was at the base camp, wrote on his blog on Sunday.

"Many of the injuries were similar to ones you might see in the [American] Midwest when a tornado hits, with contusions and lacerations from flying debris. Head injuries, broken legs, internal injuries, impalements also happened to people. Some people were picked up and tossed across the glacier for a hundred [metres]."

He continued: "People in tents were wrapped up in them, lifted by the force of the blast and then slammed down onto rocks, glacial moraine and ice on the glacier."

Rescue helicopters had begun to land at the base camp, which is used by hundreds of climbers as the starting point for Everest ascents during the main climbing season, in the morning, after the weather cleared and the sun peeped through the clouds. This gave rescuers an opportunity to ferry about 50 of the most critically wounded – climbers and sherpas – to safety.

The Xinhua News Agency reported that more than 400 mountaineers on the north side of Mount Everest were safe, quoting the sports administration of Tibet. There was an avalanche near the north side of the North Col, but it didn't hit any of them.

However, the continued seismic activities halted rescue operations.

A Danish climber, Carsten Lillelund Pedersen, wrote in a Facebook message exchange with The Washington Post that the injured had been evacuated but that the dead remained.

"It's very tragic, we have many climbers and sherpas stuck higher up in camp 1 and 2. . . . And they are getting desperate," he wrote in the message.

Another climber, Alex Gavan, tweeted that base camp had grown quiet, taking on the look of an aftermath of a nuclear blast, with "great desolation" and "high uncertainty" among those who remained.

One section was especially hard hit, a Dutch climber named Eric Arnold wrote on his blog.

"There was hardly anything left. I see very personal stuff, a log book, shampoo, slippers, reading glasses, everything."

The jittery survivors rushed out of the dining tent every time they felt a shock on Sunday, Mr Arnold wrote.

"Fear has got the better of us."

Aftershock @ 1pm! Horrible here in camp 1. Avalanches on 3 sides. C1 a tiny island. We worry about icefall team below.. Alive?— Daniel Mazur (@danielmazur) April 26, 2015

At any point during the peak climbing season, more than 1500 people could be at the Everest base camp, including climbers, sherpa guides, porters and other staff, said Eric Johnson, a Montana emergency doctor who sits on the board of Everest ER, which runs a clinic there.

It was difficult to know how many climbers were trapped on the mountain, or how many might have perished during the avalanche near or in its perilous Khumbu icefall, he said.

Sixteen sherpas were killed in an avalanche last year.

Dan Richards, the chief executive of the travel risk and crisis management firm Global Rescue, in Boston, said that his company had about three dozen mountaineering clients who had been near or around Everest at the time of avalanche.

Six were unaccounted for, and eight remained trapped on the mountain in the camps above where the avalanche started.

"They're up there. They're well supplied, and they're safe, but they're not able to descend," Mr Richards said.

His firm hoped to be able to rescue the trapped climbers via helicopter as soon as circumstances on the ground permitted, he said.

Washington Post

Follow FairfaxForeign on Twitter