At least four people have died after they were trapped underground by flood waters during a guided tour of Moscow's sewer system, local media reports.

City Mayor Sergei Sobyanin confirmed the deaths without saying how many had died, and said authorities were still searching for several missing people.

Sunday's floods, caused by heavy rain, were so rapid people could not escape.

Several companies offer tours of the city's sewers, some of which date from the 19th Century.

Russian media said the body of a teenage girl was found on Sunday evening in the Moskva river near the Zaryadye park in the centre of the city. The following morning, a man and a teenage boy were discovered in the same spot.

A fourth victim was found nearby later on Monday.

The group is said to have been exploring the underground Neglinka river, a tributary of the Moskva.

Originally more than 20 people signed up for the tour, the Ria news agency said, although most of them later withdrew because heavy rain was forecast.

Officials said eight people went ahead with the tour.

Divers are searching the Moskva river for more victims.

A criminal investigation has been launched amid allegations that safety standards were not adhered to. (BBC)