Image copyright AFP/ALPINE CLUB OF PAKISTAN Image caption Fellow climbers have branded Denis Urubko's decision "completely suicidal"

A Polish expedition hoping to complete the first winter ascent of the world's second-highest mountain, K2, says one of its climbers appears to have launched an unauthorised solo attempt.

Russo-Polish mountaineer Denis Urubko left teammates on Saturday at the base camp on the border between Pakistan and China, a spokesman told the BBC.

The 44-year-old split off from the group after a series of arguments.

One climber described a solo attempt in winter as "completely suicidal".

Expedition spokesman Michal Leksinski said he thought Urubko wanted to reach the top this month so his effort would definitely count as a winter climb.

Leksinski said he trusted his colleague to turn back if he had to, rather than endanger himself or the team. He believes the next 48 hours are critical.

Why did they fall out?

Urubko reportedly left without a radio, after refusing to discuss his plans.

"He was trying to persuade the team to push for the summit in February," a porter with the group told AFP news agency.

"He has had a heated debate with the team leader and left for the summit without saying a word."

How dangerous would a solo bid be?

Professional mountaineers have expressed dismay at the climber's decision.

"A solo attempt of K2 in winter is completely suicidal," said Pakistani climber Mirza Ali Baig.

Image copyright Getty Images Image caption K2's nickname is The Savage Mountain because it is so difficult to climb

Karim Shah, a mountaineering friend of Urubko, agreed the move was "very risky".

"He is known as the 'Himalayan expert' among the mountaineering community... but his decision is not correct and does not suit someone of his stature," he said.

How good is Urubko?

He is said to be a highly capable mountaineer who has conquered all of the world's 14 peaks over 8,000m. The expedition members believe he has probably reached an existing K2 camp located at 7,200m.

Image caption Elisabeth Revol describes her rescue from Nanga Parbat

Urubko made headlines just last month by saving a stricken French climber, Elisabeth Revol.

He and three other team members were flown by helicopter from K2 to the 8,126m Nanga Parbat in Pakistan - nicknamed Killer Mountain - where they performed an audacious night-time rescue.

K2, the pinnacle of the Karakoram range, stands at a majestic 8,611m (28,251ft). It is the only peak above 8,000m never climbed in winter.

Image copyright Reuters Image caption A spokesman for the expedition said its members would continue to support Mr Urubko

It has a higher fatality-to-summit rate than Everest, and is known as the Savage Mountain due to its fiendish conditions.

K2's steepness challenges even the world's most accomplished climbers. Avalanches are an ever-present risk, and in winter temperatures can fall to -50C, accompanied by winds of up to 200km/hr (124mph).