ISLAMABAD (Reuters) - Pakistan’s army launched a helicopter search on Thursday for a British and an Italian climber stuck on a treacherous peak known as “Killer Mountain”, Italy’s ambassador to Pakistan said.

Italian Daniele Nardi and fellow mountaineer Tom Ballard were attempting a winter ascent of the 8,126-metre (26,660-ft) Nanga Parbat in Pakistan’s Himalayas, one of the hardest mountaineering feats in the world.

Ballard is the son of famed British climber Alison Hargreaves, who became the first woman to climb Mount Everest unaided in 1995 but died that year while descending from a summit of Pakistan’s K2, the world’s second-tallest mountain.

Italian Ambassador Stefano Pontecorvo said the climbers had not been heard from since Sunday when Nardi, an experienced climber with a high profile in Italy, called his wife.

“Since then there have been three days of heavy snow,” Pontecorvo told Reuters. “We are hoping for the best.”

The weather turned on Thursday and two army helicopters were involved in the search that is expected to continue while there is daylight, Pontecorvo added.

Nardi and Ballard were attempting to climb Nanga Parbat by a route that had never been successfully used to reach the top. Two Pakistani climbers who were part of the mission had earlier abandoned the climb, Pontecorvo said.

Pakistan rivals Nepal for the number of peaks higher than 7,000 meters.

The first successful winter ascent of Nanga Parbat was made in February 2016. But a Polish climber died while trying a winter climb in January 2018, while his female French climbing partner was saved by climbers who had been airlifted from K2 to conduct a daring night-time rescue mission.

In June 2017, a Spanish man and an Argentinian were killed in an avalanche while trying to scale its peak.