Weeping and wailing as they embraced relatives on shore, survivors rescued after over an hour in the water said the two-decked, 56-year-old riverboat sank fast in a thunderstorm after listing onto its side.

Two women were confirmed dead, but as divers examined the sunken ship after dark, authorities said there was little hope of finding any of the 103 missing people alive.

"The chances of finding more survivors are very small," Emergency Situations Ministry spokeswoman Irina Andrianova said.

Andrianova told Reuters it was believed that 185 people had been aboard the riverboat and 80 had been rescued, including 77 brought to shore on another tourist boat that passed by over an hour after the sinking. Several were taken to hospitals.

The boat was heading to the Tatarstan regional capital, Kazan, 800 km east of Moscow, when it sank about 3km from shore in 20 metres of water, the emergencies ministry's regional branch said.

Reuters