A man injured in a Moscow metro derailment receives medical treatment, July 15, 2014. [Photo/IC]

At least 20 people died and scores of others were injured after a metro train derailed in Moscow during the morning rush hour on Tuesday, the Russian Health Ministry said.

"Nineteen people died on the spot, one more died later in a hospital, and 129 people have been hospitalized, with 42 of them in grave conditions," ministry spokesman Oleg Salagai told reporters.

He did not rule out more casualties as more than 1,100 people were evacuated from the scene.

The deadly incident occurred at around 8:40 a.m. Moscow time (0440 GMT) when three carriages derailed during an emergency braking in an underground tunnel in western Moscow between Slavyanski Bulvar and Park Pobedy stations.

Russian President Vladimir Putin, who is currently in Brazil, has offered "deep condolences to the relatives and loved ones of those killed and wishes a swift recovery for those injured in the crash," Kremlin spokesman Dmitry Peskov told reporters.

The president has also instructed Russia's Investigative Committee (IC) to open criminal cases over the incident.

Moscow Mayor Sergei Sobyanin has declared Wednesday as the mourning day for the victims, and pledged to bring those who were responsible to account.

The IC's spokesman Vladimir Markin has ruled out the possibility of a terror attack, saying "it is a technical incident."

"Necessary tests have been ordered and people, including Mosenergo employees, have been questioned. I do not rule out that suspects will emerge soon," Interfax news agency quoted him as saying.

Preliminary investigation suggested the accident could happen due to a false automated fire alarm which requires emergency braking.

"The most probable cause of the incident is a human factor," deputy chief of Moscow's "Means of Rescue" technological safety company Nikolai Azanov told Xinhua.

Azanov, who has participated in numerous rescue operations at railroad accidents, said the malfunctioned alarm coupled with the engine driver's incompetence could cause the crash.

"After hearing the alarm, he (the driver) likely attempted to stop the train too abruptly, which at such speed leads to derailing even at a straight section of a railway," he said.

Local authorities have said it would take one or two days to clean up the scene and restore train service in the affected area.

Tuesday's incident could become the worst technical disaster during the entire history of Moscow metro, which was established in 1935. On June 5, 2013, the Moscow metro was suspended twice in a day due to technical failures, injuring dozens of people.