* Twelve Nepali guides killed, four missing
* Ice avalanche sends house-sized blocks falling - climber
* Four-day halt called, some may abandon climb
Sherpa climbers aided by helicopters resumed a search on Saturday for four missing guides after an ice avalanche swept the lower slopes of Mount Everest, killing at least 12 in the deadliest accident on the world's highest mountain.
Climbers declared a four-day halt to efforts to scale the 8,848-metre (29,029-ft) summit and, while some decided to abandon their mission, others said they would go ahead after talking to their Nepali guides.
"I sat and counted 13 helicopter lifts - 12 were dead bodies flying overhead suspended by a long line from a helicopter," Tim Rippel of Peak Freaks Expeditions wrote in a blog.
"Everyone is shaken here at Base Camp. Some climbers are packing up and calling it quits, they want nothing to do with this. Reality has set in."
The ice avalanche struck a perilous passage called the Khumbu Icefall, which is riddled with crevasses and piled with seracs - massive ice boulders or columns that can break free without warning.
Although relatively low on the mountain, climbers say it is one of the most dangerous points on Mount Everest. There are, however, no safer paths along the famous South Col route first scaled by Sir Edmund Hillary and Tenzing Norgay in 1953.
The sherpas caught in the ice slide were ferrying equipment from Base Camp to Camp 1 - one of four waypoints that lie beneath Everest's South Face en route to the final climb to the peak.
Around 100 climbers and guides had already moved beyond the Khumbu Icefall to prepare their attempts on the summit. They are safe, but a new path will have to be made to make it possible to continue the expeditions.
Rippel's sherpas had lucky escapes - two had returned to base camp five minutes before the avalanche hit, while two were briefly trapped above the avalanche but managed to make their way down, he wrote.
The Himalayan Guides, a Nepali hiking group, said six of its sherpas had gone ahead of climbers they were accompanying in order to fix ropes and crack snow and ice to carve out a route, when they were caught in the avalanche and died.
"Now we are concentrating on the rescue. Once that is over we will hold a meeting and decide what to do next for the climbing," Bhim Raj Paudel, a member of the group, told Reuters.
His group is providing local logistics for three expeditions on Mount Everest and four teams to Lhotse and Nuptse peaks in the same region.
Lakpa Sherpa of the Himalayan Rescue Association told Reuters from the tented base camp near the incident site that a ground search party would begin climbing in clear weather on Saturday morning.
"We have two helicopters stand by in the area and will start looking for those who are still missing. Many of them have already been rescued," Nepali Army spokesman Brig. Gen. Jagdish Chandra Pokharel told Reuters.
The fatal incident was the result, climbers said, of a so-called ice release and not a more typical snow avalanche.
"When the serac hovering off the West Shoulder of Everest collapsed, it sent house-sized ice blocks all over the route," Alan Arnette, a climber and motivational speaker, wrote in an updated blog post from Base Camp.
"With hanging ice, it may stay there for decades or fall tomorrow - there is no way of knowing or predicting."
Californian climber Adrian Ballinger described the Khumbu Icefall as the "corridor of calamity" in a video shot on a past expedition that showed massive ice blocks overhanging his path.
"It's always the most dangerous part of the mountain to climb, because the ice is constantly moving, there's so many crevasses and seracs where you need to use ladders and ropes to get through the very technical terrain," he told Reuters.
Above stands a feature called the West Shoulder that can shed avalanches on a "pretty regular basis", said Ballinger, who works for Alpenglow Expeditions and was preparing to depart for Nepal.
"Whenever we're in it, and whenever anyone's in it, we're very conscious of moving as quickly and as efficiently as possible because there is this sort of uncontrolled risk."
Sherpas often make 20-25 round trips to carry kit and supplies to advanced camps, exposing them to greater risk. The most endangered are the so-called Icefall Doctors - a team that maintains and fixes the route.
It was first major avalanche of this year's climbing season on Everest, which has been scaled by more than 4,000 climbers. Some 250 climbers have died on the mountain, which is on the border between Nepal and the Chinese region of Tibet and can be climbed from both sides.
Expedition leaders reported that there was anger among some guides after the government announced immediate payments of $400 to the victims' families to cover funeral costs. Insurance cover typically amounts to $5,000.
"I hope the Ministry of Tourism will take care of their families," said Arnette. "I know the expedition companies will provide support. But in an event of such magnitude, a reaction of similar order is in order by the Nepal government."
A rising number of tourists has raised concerns about safety and environmental damage, although Nepal still plans next year to cut fees for those wishing to do the trek.
The government has issued permits to 334 foreign climbers this season, up from 328 for the whole of last year. An equal number of guides also climb to help the foreign mountaineers.
"The atmosphere at Base Camp is now of shock and of grieving," Scottish film maker Ed Wardle told Britain's Channel 4 News on Friday night, adding that "many of the expeditions here will pack up and go home".
"For this number of people to die at the very beginning of the season is completely unacceptable," he said. "We came here looking for adventure, to celebrate Everest, but for something like this to happen makes the whole thing seem pointless."