Two people were killed and at least three injured by an explosion on the beach in the Olympic host town of Sochi along the Black Sea, Russian agencies reported.

"At 10:35 am an unidentified device exploded at Loo beach, two people were killed," the head of the city's emergency situations ministry, Anatoly Sherbinin, was quoted by news agency Interfax as saying.

The victims were a 31-year-old Russian and a 22-year-old tourist from Ukraine, an unidentified law-enforcement official was quoted by the agency as saying.

Reports said people had been evacuated from the beach and an investigation was under way. No other details were given as to the cause of the blast.

Russian President Dmitry Medvedev interrupted his vacation to instruct his envoy to the region to take "all necessary measures" and personally oversee the investigation, Kremlin spokeswoman Natalya Timakova said Thursday.

Sochi is a favourite Russian tourism destination and has seen huge investments and a construction boom since winning the bid to host the 2014 Winter Olympic Games.