At least 20 people were killed and more than 120 were injured this morning when a Moscow underground train derailed during rush hour in one of the worst accidents on the Russian capital's subway system in years.

President Vladimir Putin, who is away in Brazil, ordered a criminal investigation into the accident, which is likely to raise further questions over Russia's transport safety record.

There was no suspicion of a militant attack; the cause of scores of deaths in Moscow's subway system in years past.

Three carriages derailed on a train travelling between two stations at 70km/hr at around 8.35am (5.35am Irish time) on the world's busiest subway system.

Rescue workers evacuated more than 1,000 people from the area of the accident, the Emergencies Ministry said.

Injured passengers were carried on stretchers out of both metro stations.

Helicopters took the most seriously hurt to hospital.

"There is no one alive left," Moscow deputy mayor Peter Biryukov said. "The cause is not known, the work continues."

Some bodies were recovered from the wreckage but others remained underground, crushed in the accident, he said.

The Investigative Committee put the toll at 20 dead.

Russian news agency Itar-Tass quoted Health Ministry representative Oleg Salagai as saying that 129 people were injured, 42 of whom were in a serious condition.

The Investigative Committee, which answers to Mr Putin, said it had opened a criminal case on suspicion of failure to meet safety guidelines, but that it had not yet determined the cause of the accident, although no militant involvement was suspected.

A power surge may have caused the train to stall and several cars to come off the rails between the Slaviansky Boulevard and Park Pobedy stations, the Investigative Committee said.

Russians regularly criticise the country's transport safety record.

Recent disasters included the 2011 sinking of a ferry boat that killed 128 and a plane crash that killed the Lokomotiv Yaroslavl ice hockey team the same year, both of which were blamed on lax safety regulations and technical errors.

On weekdays as many as nine million people use the subway system.

Famed for its high-vaulted halls adorned with Soviet socialist realist art, the underground network has expanded from 13 stations opened in 1935 to 194 stations across the megalopolis today.

Islamist militants have previously carried out deadly attacks in Moscow, including twin suicide bombings that killed 40 people on the subway in 2010.