Two people have been killed and seven others injured by an explosion on a beach in Sochi, Russia's popular Black Sea resort, which will host the 2014 Winter Olympic Games.

The blast occurred when visitors touched a device on the beach, a spokesperson for the regional branch of Russia's interior ministry said.

A man and a woman died on the spot and three other visitors, including an eight-year-old child, were wounded on the beach, which is 15km north of the centre of Sochi.

Other sources had higher injury totals.

An unidentified police source was quoted as saying 13 people were injured.

Local authorities said they quickly evacuated visitors from all Sochi beaches and checked them for more explosives. None were found.

Dmitry Medvedev, Russia's president, has ordered his envoy in the region to oversee an investigation. 

Reward offered

An interior ministry official said the explosion could be part of a battle for control over the area between local criminal groups.

The local police have offered a reward of $85,000 for information that would help authorities track down the perpetrators.

Sochi is one of the most fashionable Russian resorts, favoured by the country's elite and Russian presidents who use it as a holiday destination.

The government has allocated over $10bn to reconstruct the resort before the 2014 Winter Olympic Games, which has attracted businesses but also criminal groups.

Sochi is situated just kilometres from the border with Georgia's breakaway region of Abkhazia, where rising tensions between Moscow-backed separatists and Tbilisi have fuelled fears of a possible war.