Russian rescue officials on Sunday played down the chances of finding new survivors after more than 100 people went missing when their cruise boat sank in a Volga River storm.

"Forty-seven emergencies ministry divers are inspecting the craft," an emergencies ministry official told the Interfax news agency.

"The results of the inspection show that the chance of us finding survivors are minimal," the official said.

Rescuers have confirmed saving at least 77 people off the boat, which was believed to be carrying more than 180 tourists and crewmembers when it sank in a heavy storm on Sunday afternoon.

Two people have been officially confirmed dead.

A passing riverboat managed to pick up 76 people off the Bulgaria, which was packed with 188 tourists and crew members when it went down during a storm in the central Russian republic of Tatarstan, rescue officials said.

"The fate of 111 people remains unknown," local emergencies ministry spokeswoman Svetlana Lebedeva said by telephone.

But at least one survivor said the boat sank and capsized so quickly he feared many people had died, including dozens of children.

The accident happened at around 2:00 pm (1000 GMT) some three kilometres (two miles) from the shore, officials said. The Bulgaria now lying 20 metres (66 feet) down on the riverbed, they added.

The first survivors were picked up by a riverboat, the Arabella, which was the first vessel to arrive at the scene.

Officials said there was still hope for more survivors because the wide bend of the river where the ship went down had 13 islets to which people could swim.

But one of those rescued said he suspected many people -- including dozens of children -- might have died. An unnamed survivor told Interfax on his rescue that around 30 kids had gathered in a playroom on the second deck moments before the accident.

"I fear they all died," the person said.

He described how the ship had sunk within minutes after suddenly leaning on its right side and capsizing.

"It flipped over in three minutes and sank. Lots of people died."

And a school director from the village of Syukeyevo, near the scene, told the RIA Novosti news agency: "The river was fairly calm, but there was a heavy rain and storm,"

State news reports said the two-deck vessel was built in 1955, in what was then Czechoslovakia, though Russian television said it had recently been modernised and included comfortable cabins for up to four people.

Some of the modernised craft, however, are equipped with only two rescue boats -- and not the four the original models came with, RIA Novosti reported.

Russian President Dmitry Medvedev spoke by telephone to his emergencies minister and the local head of the administration after learning of the disaster, the Kremlin said.

More than 80 rescue workers were rushed to the region, with a Mi-8 helicopter surveying the scene from the air, the emergencies ministry said.

The Volga River has remained a popular summer tourist destination since the Soviet era, with occasional accidents rarely leading to major fatalities.

The last shipping accident recorded by Russian state media occurred in September 2010, when seven people were killed on a lake above the Arctic Circle.

Perhaps the most famous shipping disaster occurred in August 1986, when the Admiral Nakhimov collided with a cargo ship while leaving a bay on the Black Sea.

Soviet reports said the boat sank within eight minutes, claiming the lives of 423 people.