MOSCOW, July 15 -- A Chinese citizen was confirmed dead after a metro train derailed in Moscow at the height of the morning rush hour on Tuesday, Chinese embassy authorities said.

The Chinese male citizen, whose identity was to be further verified, was killed when three carriages derailed during an emergency braking in an underground tunnel in western Moscow between Slavyanski Bulvar and Park Pobedy stations, a Chinese consular official told Xinhua.

The official said that the embassy was verifying information about any potential rise of casualties among Chinese.

Russian Health Minister Veronika Skvortsova said the incident has claimed at least 21 lives and injured some 200 more.

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

Russian President Vladimir Putin, who is currently in Brazil, 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. IC spokesman Vladimir Markin has ruled out the possibility of a terror attack, saying "it is a technical incident."

Tuesday's incident could become the worst technical disaster in the 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.