The statement by Admiral Viktor Fyodorov to the NTV television channel followed a day of desperate rescue efforts and widely varying estimates of how much oxygen remained on the tiny vessel.
A British military plane carrying a sophisticated unmanned underwater rescue vehicle took off for the disaster scene in Russia's Far East late on Friday and the US Navy was scrambling to send another.
Both could reach the site off the Kamchatka Peninsula within time - if earlier estimates that there was enough oxygen to keep the seven alive for 24 hours held true.
However, Fyodorov said early on Saturday that there was oxygen for "at least 18 hours," a distinctly less optimistic statement than his earlier assertion that the air would last into Monday.
Along with earlier contradictory statements about whether the sub had been hooked, the confusion over the air supply darkly echoed the sinking of the nuclear submarine Kursk almost exactly five years ago.
Putin has yet to make a public
comment on the accident
That disaster, in which all 118 aboard died, shocked Russians and deeply embarrassed the country by demonstrating how Russia's once-mighty navy had deteriorated as funding dried up following the 1991 Soviet collapse.
The new crisis underlined that promises by President Vladimir Putin to improve the navy's equipment have apparently had little effect. Authorities initially said a mini-sub would be sent to try to aid the stranded one, but the navy later said the vehicle wasn't equipped to go that deep.
Putin was sharply criticized for his slow response to the Kursk crisis and reluctance to accept foreign assistance.
By late Friday, Putin had made no public comment on the latest sinking.
But in contrast to the Kursk incident, Russian officials asked for outside assistance on Friday within hours of news breaking about the sinking - instead of waiting until hope was all but exhausted as they did in 2000.
The United States and Britain were scrambling to send unmanned underwater rescue vehicles called Super Scorpios to the scene and Japanese ships also were rushing to the area.
The Kursk incident sparked calls
to improve the military
The British vehicle was expected to arrive first and was due to land in the city of Petropavlovsk-Kamchatsky - the regional capital - at about 5am Moscow time on Saturday, spokesman Anton Atrashkin said.
Airlifting a US underwater vehicle to the area marks the first time since the World War II era that a US military plane has been allowed to fly there.
The mini-submarine was trapped in Beryozovaya Bay, about 75km south of Petropavlovsk-Kamchatsky, the capital of the peninsular region in Russia's far east.
Initial reports said it had become ensnared in a fishing net, but Fyodorov later said it also had become caught on an underwater antenna.
The vessel, which was launched from a ship in a combat training exercise, was too deep to allow the sailors to swim to the surface on their own or for divers to reach it, officials said.
The sailors were in contact with authorities and were not hurt, Pacific Fleet spokesman Captain Alexander Kosolapov said in televised comments.
The Russian military has suffered
from a lack of funds since 1991
After the Kursk disaster, Putin had called for serious improvements in the military's equipment and training, but little improvement has been noticed publicly.
Although the navy reportedly ended its deep-sea diving training programs due to funding shortages a decade ago, it does have a device called the Kolokolchik, essentially an updated diving bell, that can be used for some underwater rescues.
However, the mini-sub lies so deep that the device apparently would be useless.
The trapped AS-28, which looks like a small submarine, was built in 1989. It is about 13.5 meters long and 5.7 meters high and can dive to depths below 500 meters. A vessel of the same type was used in the rescue efforts that followed the Kursk disaster.