At least 18 people have been killed in southern Russia after a bus carrying construction workers plunged into the sea.

Eight more people were injured, five seriously, and 16 others were rescued after the coach fell from a pier on Friday morning, said Russia's emergency ministry.

The vehicle had been carrying about 40 workers who were building docks for an oil company on the Black Sea coast close to Crimea.

At least 20 were rescued and divers have been searching the waters underneath the pier for missing men.

The ministry has has not yet established what caused the crash, which happened at about 8am near Volna, a village in the region Krasnodar.

A criminal investigation has been launched, Russian news website Life reported.

"All possible versions of the accident are examined, including those related to a violation by the driver of the traffic rules or a technical failure," said the Russian Investigation Committee.

Rescuers work near a pier after a bus carrying a group of workers plunged into the Black Sea (Reuters) (REUTERS)

The crash may have been caused by a failure in the vehicle's braking system, said Oteko Portservice, the company which runs the shipment facility.

The passengers on the bus were reported to have been working on the construction of tanker docks for Russian energy on company Tamanneftegaz, one of several firms drilling for oil and gas off the Black Sea coast.

The bus has since been lifted from the water by a crane.