The bus fell into an ice fissure three metres wide and sank, the regional branch of Russia's emergency situations ministry said in a post on social media.

"Rescuers carried out an inspection using an underwater camera. The bodies of seven people were discovered," it said.

The nationalities of the victims was not explicitly stated, though the governor of the Irkutsk region said they were foreign citizens and that "the consulate general of the PRC (People's Republic of China) has been notified."

Governor Igor Kobzev said according to eye witness reports one tourist managed to escape the bus before it sank.

The vehicle was found 18 metres beneath the surface.

Russia's Lake Baikal, the deepest in the world, is a major tourist destination in Siberia.

It is just north of Mongolia and popular with Chinese visitors.

In the winter, the vast body of water freezes, creating visually stunning crystal-like columns and a distinctive cracking noise from crashing waves trapped underneath the surface.

Officials set up specific ice roads over the frozen lake that are monitored by authorities and opened for certain types of vehicles depending on the conditions.

Driving elsewhere on the lake is officially prohibited.

Kobzev said the road had not been cleared for use and that a criminal case had been opened into the incident.