Planes carrying US and UK rescue teams and gear have arrived in the Russian Far East to join efforts to save seven sailors trapped in a mini-submarine.
Hopes now hang on the unmanned Scorpio craft that can work at depth
Sophisticated underwater equipment is being taken to the site of the accident off Russia's Kamchatka peninsula.
The Russian craft had been on a training exercise when it got snagged and trapped underwater on Thursday.
A Russian naval spokesman now says rescuers have 24 hours to reach it before its oxygen supplies run out.
The craft sank too deep to allow the sailors to swim to the surface on their own and - at that depth - only another submersible such as a Scorpio can release them.
The Scorpios supplied by the US and Britain can work unmanned to cut through thick cable and metal, but they will take several hours to get the actual site of the accident.
The Russian navy has so far failed in its rescue attempts.
A Russian Pacific Fleet chief said a cable attached to the mini-submarine by a rescue vessel had failed to move the sub.
Maximum speed: 3.3 knots
Maximum depth: 1,000m
Air supply: 120 hours
As many as nine Russian ships have been sweeping the sea bed to try to dislodge the Priz mini-submarine, which sank to a depth of 190m (620ft) after the accident.
Japanese vessels are also on their way.
There have been contradictory reports about the cause of the accident and how much oxygen is left on the Russian vessel.
Naval officials said they were keeping in touch with the crew through acoustic signals. All were said to be unharmed.
Echoes of past
The BBC's Sarah Rainsford in Moscow says the confusion surrounding the rescue is making the scenes eerily familiar.
Max depth: 1,520m
Tools: Cable cutter and two lifting arms
Almost exactly five years ago, the Kursk submarine sank after explosions on board, killing all 118 men on board.
This time too, it took more than 24 hours before the navy admitted to an accident and longer still to ask for help, our correspondent says.
Some of the sailors survived the initial blast, but ran out of oxygen. The authorities then were accused of a cover-up.
The AS-28 'Priz' mini-sub can operate at depths of 1,000m, but 190m is too deep to allow the crew to attempt escape