As many as 21 cargo ships may have become stranded in ice in November after attempting to follow the Northern Sea Route along the Russian Arctic coast.
Accounts vary, but according to the Norwegian Barents Observer website, six were stranded near the north Chukotka coast, a remote region near the Bering Strait.
It said 15, including bulk carrier cargo ships and an oil tanker, were stuck in the East Siberian Sea.
Vedomosti, a Moscow-based business publication, said at least 18 vessels were stranded.
The Northern Sea Route has become more easily navigable in recent years as global warming results in thinner ice.
The route appears to have frozen earlier than expected this year. Ships began reporting being stranded in early November.
Alexei Likhachyov, the director general of Rosatom, a nuclear energy company that runs a fleet of nuclear-powered icebreaker ships, said that weather forecasts were inaccurate.
Shipping companies using the route typically book icebreakers to accompany them if they are expecting bad weather.
Last week, the 14,000-tonne Russian diesel-electric icebreaker Novorossiysk began pushing its way through the Chukchi Sea to assist stranded ships.
According to the Vyborg Shipyard company, which built the vessel, the Novorossiysk is capable of smashing ice 1.5 metres thick.
A perilous shipping route
Records show that ships getting stuck in polar ice was often a disaster. Crews would have to wait for sea ice to melt, with the risk of running low on supplies, while in some cases the ice could crush the hulls of the vessels.
Famously, this was the fate of British Antarctic explorer Ernest Shackleton's three-masted ship Endurance, which was crushed by ice in 1915.
This forced the crew to survive for 500 days as teams trekked across polar ice or rowed a seven-metre lifeboat, the James Caird, across the Southern Atlantic to a whaling station on the island of South Georgia.
While modern ships are far stronger, bigger, better insulated and more powerful, becoming ice-locked can still be dangerous.
In December last year, a Russian military transport ship was stranded in Arctic ice near the port of Arkhangelsk, nearly running out of water and fuel before being rescued by the nuclear-powered icebreaker Kigoriak, which smashed through metre-thick ice to rescue the vessel.
The present crisis could be one of the worst incidents of ships getting stuck in recent years.
In 2014, more than 50 explorers had to be rescued by helicopter when their ship, Akademik Shokalskiy, was icebound for 10 days off the coast of Antarctica.
