Health Minister Veronika Skvortsova said the patients were in "a bad condition with burns, with multiple injuries typical of blast-induced wounds".

The blast comes a day after 17 people died in another suicide attack at the central station in the city.

President Vladimir Putin ordered tighter security nationwide after the morning rush-hour blast.

Moscow is concerned that militants could be ramping up violence in the run-up to the 2014 winter Olympic Games in the city of Sochi in February

Investigators said they believed a male suicide bomber set off the blast, a day after a similar attack killed at least 17 in the main rail station of a city that serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian Seas and the Caucasus mountains.

The bomb used was packed with "identical" shrapnel to that in the rail station, indicating they may have been made in the same place and supporting suspicions the bombings were linked, said Vladimir Markin, a spokesman for the investigators.

RA/NJF