The Russian navy is racing against time to rescue the seven crew of a deep-sea diving vessel trapped on the ocean floor in the Russian Far East.
The Priz submersible got caught in a fishing net and sank to a depth of 190m (620ft) off the Kamchatka peninsula on Thursday, officials say.
Russian ships are sweeping the area to try and free the submarine and the US, Britain and Japan are sending help.
A navy spokesman said the sailors had enough air for only another 24 hours.
Earlier Russian officials had said the sailors might have up to another four days' supply.
Capt Igor Dygalo told Russian television the crew had been told to keep physical activity to a minimum and wear standard issue outfits to keep warm.
But he said "there was no panic on board".
Earlier, he said: "There is air remaining on the underwater apparatus for a day - one day."
While two Russian ships were sweeping the seabed in an attempt to cut the net caught around the propeller, the US, Britain and Japan were mobilising their vessels.
Japanese officials said four vessels carrying rescue equipment were on their way, but would take at least three to four days to reach the site.
Maximum speed: 3.3 knots
Maximum depth: 1,000m
Air supply: 120 hours
The US navy said a Super Scorpio unmanned vessel was being flown out from San Diego on Friday morning Pacific time (late afternoon GMT).
Once in Russia, it would be loaded onto a Russian ship and taken to the site.
Britain is also sending a naval Scorpio vessel from Scotland's Prestwick airport on Friday night aboard a Royal Air Force Boeing C-17 transport plane.
It is due to arrive at Petropavlovsk-Kamchatsky at 0500 Moscow time (0100GMT) on Saturday, a British embassy spokesman in Moscow said.
Both vessels have remote-controlled arms capable of cutting free the submarine, but it was unclear whether either would make it in time.
Capt Dygalo said that, while they waited for international back-up, "a plan is being drafted relying on our own resources".
"The operation will continue non-stop until a result is reached," he told NTV television.
The stricken submersible, normally used for underwater rescue operations, was taking part in a military exercise when its propeller became entangled in the net on Thursday.
Officials said there was "technical communication" with the sailors, but no voice contact. None is believed to be hurt.
The vessel is too deep to allow the sailors to swim to the surface on their own, and another submersible is needed to release them.
The accident occurred almost exactly five years after the nuclear submarine Kursk sank to the bottom of the Barents Sea after explosions on board.
All 118 men on board died - including some sailors who survived the initial blast but ran out of oxygen.
Russian authorities were sharply criticised for their handling of the crisis.
The AS-28 'Priz' minisub can operate at depths of 1,000m, but 190m is too deep to allow the crew to attempt escape