Two people died in an explosion on a tourist beach in the Olympic host city of Sochi, drawing a strong reaction from authorities, Russian news agencies reported.

"At 10:35 am (0600 GMT) an unidentified device exploded at Loo beach, two people were killed and 13 injured including an 8-year-old girl," Interfax quoted an unidentified law-enforcement officer as saying.

A 22-year-old woman and her male companion, 31, were killed after she picked up a bag that exploded on the wharf alongside the beach, agencies cited witnesses as saying.

Agencies said the woman was a Ukrainian citizen.

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

Sochi's interior ministry announced a rare 2-million-ruble (85,000 dollar) reward for information about the crime.

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

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.

The resort on the Black Sea coast remains quiet and affluent despite its proximity to the violence-wracked provinces of Chechnya and Ingushetia in the Russian Caucasus.