"I strongly condemn and express my my condolences and the Italian state's closeness to the Russian state and people for the attack a few hours ago in St Petersburg," Foreign Minister Angelino Alfano said in a statement.
"Italy will continue to support Russia in the fight against terrorism and for peace and international security," he added.
The bomb blast hit a train between Sennaya Ploshchad and Tekhnologichesky Institut stations at about 2.30 local time.
Russian President Vladimir Putin said all causes, especially terrorism, were being investigated. He was in St Petersburg at the time of the blast, his spokesman Dmitry Peskov said.
The discovery of an explosive device at another station, Ploshchad Vosstaniya, indicates the attack was deliberate, and reports in Russia say the explosion came from a briefcase left on the train.
Several transport hubs in Russia have been attacked in the past. In 2010, 38 people died in a double suicide bombing on the Moscow metro.
A year later, a bomb exploded on a high-speed train travelling between Moscow and St Petersburg, killing 27 and injuring another 130.
Both attacks were claimed by Islamist groups.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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