Story highlights Four people remain missing, Nepal Tourism Ministry official says

The deadliest year on Mount Everest was 1996, when 15 people died

More than 300 climbers have been given permission to tackle Everest this spring

About 400 Sherpas will help them make the grueling ascent

A high-altitude avalanche Friday killed 12 Sherpa guides and seriously wounded three in the single deadliest accident on Mount Everest, officials said.

Four others are missing, said Madhu Sudan Burlakoti of Nepal's Tourism Ministry, adding that six people were injured in total.

A group of about 50 people, mostly Nepali Sherpas, were hit by the avalanche at more than 20,000 feet, said Tilak Ram Pandey of the ministry's mountaineering department.

The avalanche took place just above base camp in the Khumbu Ice Fall.

The climbers were accounted for, Pandey said. "Rescue teams have gone ... to look for the missing."

JUST WATCHED Deadliest accident on Mount Everest Replay More Videos ... MUST WATCH Deadliest accident on Mount Everest 02:50

JUST WATCHED Climber: Avalanche victim was my equal Replay More Videos ... MUST WATCH Climber: Avalanche victim was my equal 03:42

JUST WATCHED Deadliest day on Mount Everest Replay More Videos ... MUST WATCH Deadliest day on Mount Everest 01:47

Photos: Everest: from base camp to the summit Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Mount Everest is a journey that challenges human nature on every level. Sandra LeDuc captured this photo as climbers approached the Hillary Step, before the Everest summit. Click through our gallery to see more photos from climbers taken on Everest during 2012. Hide Caption 1 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Yak trains are used to carry the pounds of gear it requires to climb Everest. Jon Kedrowski's photo captured these yaks on the way to base camp. Hide Caption 2 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – A helicopter flies into base camp. Rescue helicopters can't fly above Camp 2 on Everest to reach stranded climbers. Hide Caption 3 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – A photo shows Jon Kedrowski crossing a crevasse in the Khumbu Icefall on a ladder. Hide Caption 4 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Jon Kedrowski snapped this photo at 6:30 p.m. on May 19th, 2012, at 26,000 feet as he looked out from his tent. He thought the climbers were beginning their summit attempt, but many clustered at the top were just beginning to come down from the top. Hide Caption 5 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – The tents from Camp 1 appear incredibly small against the backdrop of Everest. Hide Caption 6 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Nadav Ben Yehuda (left), Aydin Irmak and a Sherpa after Ben Yehuda rescued Irmak. Hide Caption 7 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Ben Yehuda's hand after frostbite set in. Hide Caption 8 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Ben Yehuda's photo of a lone tent at Camp 3 shows the loneliness that can reign on Everest. Hide Caption 9 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – The peak of Nuptse, just over a mile southwest of Everest, is visible amongst the clouds in the Nepalese Himalayas. Hide Caption 10 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Climbers on Everest work to help someone in distress. Hide Caption 11 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Sandra at Everest base camp with the team Sherpas. Hide Caption 12 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – The mesmerizing view from Everest's Pumori Camp 1, showing the visual effects of the atmosphere. Hide Caption 13 of 14 Photos: Everest: from base camp to the summit Everest: From base camp to the summit – Sandra LeDuc captured this photo of a triumphant and relieved Jon Kedrowski reaching the summit of Mount Everest on May 26, 2012. Hide Caption 14 of 14

Before Friday, the deadliest single-day toll was from an accident in May 1996, when eight climbers disappeared when a huge storm hit. Their tragic story was chronicled in Jon Krakauer's bestselling book "Into Thin Air."

Readying for the climb

Between May 15 and 30 is usually the best window for reaching the 29,028-foot peak.

Climbers and guides had been setting the ropes for the route, acclimating to the climate and preparing the camps along the route when the avalanche hit Friday, said Gordon Janow with Alpine Ascents International in Seattle.

Climbers arrive in April to acclimate to the altitude before heading toward the summit of the world's highest mountain.

Ethnic Sherpas acts as guides for the mostly foreign clients.

Busiest season

The spring climbing season is the busiest of the year.

About 334 foreign climbers have been given permission to climb Everest over the next couple of months, with an estimated 400 Sherpas helping them, mountaineering official Dipendra Poudel said.

Until the late 1970s, only a handful of climbers reached the top each year. The number topped 100 for the first time in 1993. By 2004, it was more than 300. In 2012, the number was more than 500.

The deadliest year on Everest was 1996, when 15 people died. Another 12 climbers were killed in 2006.