At least four people have died and 49 are missing after an oil rig capsised and sank in icy seas in Russia's far east in the country's second major water disaster since July.
Another 14 people were rescued as officials clung to hope that more could be pulled out alive even after spending some nine hours in the freezing waters of the Okhotsk Sea.
But rescue efforts have been hampered by high winds and waves up to five metres high.
President Dmitry Medvedev ordered a probe into the disaster and told officials to provide victims with all necessary assistance, the Kremlin said, adding that the operation was taking place in "complicated weather conditions".
The Kolskaya oil rig platform with 67 people on board was being towed by a tugboat and an icebreaker from the Kamchatka peninsula towards Sakhalin island when it got caught up in a storm.
The crew was waiting to be evacuated by helicopter but the platform capsised and sank before they could board their rescue rafts, regional ministry spokesman Taimuraz Kasayev said.
"The portholes were damaged by ice and waves, and water began going into the vessel," Kasayev said.
The rig was fully submerged by Sunday evening, and all four life rafts have been found with no one on board.
It was 200 kilometres off Sakhalin when the accident occurred at 0145 GMT (01:45am NZT) on Sunday in high winds with temperatures of minus 17C.
"The investigation is looking into safety regulation violations during the towing of the platform and a disregard of poor weather conditions as the main reasons for the incident, as there was a strong storm in the area," the Investigative Committee said in a statement.
Anton Prokhorov, an official with the emergencies ministry, declined to say whether there was any hope of finding any more of the missing alive.
"It all depends on weather conditions," he said. "People can stay alive in the water for around six hours ... then hypothermia begins," he told AFP from the city of Yuzhno-Sakhalinsk, stressing however that instances had been recorded of people surviving in frigid water for longer than six hours.
The federal sea and river agency said in a statement that four people in wetsuits were found in the water without any signs of life but rescuers were unable to pull them out because of the poor weather.
Emergencies officials said two of the rescued people were suffering from hypothermia and another two had minor injuries.
Kasayev said the Kolskaya oil rig had experienced technical problems even before the accident, and had been forced to constantly pump water out of one of its air tanks due to a leak.
The latest disaster comes after 122 people drowned in the Volga river in July when an overcrowded pleasure boat sank in central Russia after capsising in stormy weather.
The Kolskaya rig belonged to state-owned company Arktikmorneftegazrazvedka (AMNGR) based in the northern city of Murmansk.
It was engaged in shelf exploration in the Kamchatka peninsula for the Gazflot company, a subsidiary of Russia's gas giant Gazprom.
The risk of an oil spill from the platform is "minimal", emergencies ministry official Kasayev said. "The fuel is in air-sealed tanks, so there is no risk of a spill."