The crew was thought to have enough oxygen until Friday, although the Russian navy has now revised that estimate to nine days - until 25 August.
A British mini-submarine and team of 27 rescuers has been flown out to help Russian teams and Norway is sending a diving ship and crew of 12 to 15 divers.
The Russians say there has been no sign of life aboard the submarine on Wednesday, but that this did not mean that there were no survivors.
The crew was thought to have enough oxygen until Friday, although the Russian navy has now revised that estimate to nine days - until 25 August.
'Critical' situation
Attempts by rescue capsules to latch on to one of the cargo hatches of the submarine have been frustrated because of strong currents and swirling sand.
"Sand and silt envelop the capsule causing zero visibility," said a navy officer, who declined to be named.
Moreover, the submarine is lying at a sharp angle, making any
latching operation more difficult.
One capsule was almost lost during an earlier attempt because of the strong currents.
Now the Russians have begun using a larger version, called the Bester, a 50-tonne machine manned by a crew of two or three and capable of carrying up to 20 evacuees.
'Underwater helicopter'
The sending of the British rescue sub came as Russian authorities, in an about-turn, finally decided they would ask for foreign help.
Russian President Vladimir Putin earlier called the situation "critical".
An aircraft carrying the mini-sub and a team of 27 rescuers arrived at Trondheim in Norway on Wednesday afternoon, en route to Murmansk.
The LR5 sub is a 33ft-long craft described as an "underwater helicopter", with a sophisticated camera system and an array of debris-clearing tools.
Its aim would be to hook up with the Kursk's hatch, but it was not clear whether the British craft's fittings would be compatible with those of the Russian navy.
It could also take several days to get the British equipment out to the site of the accident and bad weather could hamper the British team's efforts just as it has hindered Russia's.
A high-level Russian military delegation will talk to Nato on Thursday in Brussels about the rescue operation.
Russian fears
Across Russia, there is growing unease at what is perceived as potentially one of the country's worst maritime disasters in recent times, and criticism of the military's slowness at asking for help from abroad.
"I heard that the boat is on the bottom of the sea. They can't raise it themselves, but they're too ashamed to ask anyone, Britain or France, for help. The accident itself is a sign of Russia's decay," one Russian told the BBC earlier on Wednesday.
Analysts say that apart from injured military pride, there may be security worries that led Moscow to hesitate before calling for help.
Independent military analyst Pavel Felgenhauer says that the Russian navy suffers from a pervasive culture of secrecy.
The submarine - one of the biggest and newest in the Russian navy - has been lying on the sea bed since Saturday at a depth of more than 100 metres.
An explosion is believed to have ripped through a torpedo tube during a military exercise.
Flooding has closed down the front sections of the boat, forcing the survivors to the back.