Donald Trump has commented on the St Petersburg subway bombing attack, describing it as "a terrible thing"

"Happening all over the world," the President said, "absolutely a terrible thing."

He was sitting in a meeting with Vice President Mike Pence, senior US officials, and Egyptian President Abdel Fattah al-Sisi, who is on his first official state visit to the White House.

Russian President Vladimir Putin has said the blasts in the St Petersburg metro could be terror attacks. Authorities are still investigating the incident that has left at least 11 dead and 50 injured but are considering terrorism "first of all", Mr Putin said in a statement.

Mr Putin happened to be visiting the city the day of the attack when an unidentified explosive device went off on a train travelling from Sennaya Ploshchad station to the Technology Institute station, Russia's National Anti-Terrorist Committee said.

A second device was found by authorities at the city's busiest station, Ploschad Vosstaniya station.