At least 10 people were killed and 47 injured when an explosion tore through a train carriage in the St. Petersburg metro on Monday, Russian authorities said, in what the country's general prosecutor has officially declared a terror attack.

Interfax news agency quoted unnamed sources as saying that the explosive device had been left in a briefcase on the metro carriage and had been filled with shrapnel. Russian media released photos of the suspected bomber.

>> St. Petersburg metro attack: In photos | Russian Media Releases Photos of Suspected St. Petersburg Metro Bomber >>

Russia's anti-terrorism committee said a second explosive device was found and detonated at a different location from where the earlier blast took place. All St. Petersburg metro stations were closed following the incident and the Moscow metro said it was taking unspecified additional security measures in case of an attack there.

When asked about the blast, U.S. President Donald Trump told reporters the incident was a "terrible thing – happening all over the world – absolutely a terrible thing."

Open gallery view Blast victims lie near a subway train hit by a explosion at a metro station in St. Petersburg, Russia, April 3, 2017. Credit: /AP

Prime Minister Benjamin Netanyahu offered condolences on Monday evening to Russian President Vladimir Putin and the families of those killed in the blast. "The citizens on Israel stand by the Russian people on this difficult day," Netanyahu said in a statement.

Putin, St. Petersburg's former deputy mayor, who was himself in the city for a meeting with Belarussian leader Alexander Lukashenko, said he was considering all possible causes for the blast, including terrorism, and was consulting with security services.

Russia's state investigative committee said it had opened a criminal case over the bombing on charges of a terrorist act. The agency, which has sweeping powers, said it had sent a group of investigators to St. Petersburg, adding it would consider other possible causes of the incident besides terrorism.

Interfax quoted a security source as saying that Russia issued search warrants for two people for their possible role in the blast. One of the suspects "is thought to have placed the explosive device in the metro wagon and the second person [is sought] for leaving a bomb at the metro station 'Ploshchad Revolutsii'," the source told Interfax.

Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station after the blast occurred. A helicopter hovered overhead as crowds gathered. A huge hole was blasted in the side of a carriage with metal wreckage strewn across the platform.

Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces. Passengers were seen hammering at the windows of one closed carriage.

St. Petersburg emergency services first said that two explosions had killed at least 10 and injured 50, but a source in the services later said that there had been only one blast that occurred in a tunnel between stations, killing at least nine and injuring 20. Russia's Health Ministry has since raised the casualty toll, saying 10 people were killed and 47 were injured.

The blast occurred at 2.40 p.m., well shy of the evening rush hour.

Russia has been the target of attacks by separatist Islamist Chechen militants in past years. ISIS, which has drawn recruits from the ranks of Chechen rebels, has also threatened attacks across Russia in retaliation for Russian military intervention in Syria.

Russia's air force and special forces have been backing Syrian President Bashar Assad in fighting rebel groups and ISIS fighters now being driven out of their Syrian strongholds. Moscow has been on particular alert against Chechen rebels returning from Syria and wary of any attempts to resume attacks that dogged the country several years ago.

In 2010, at least 38 people were killed when two female suicide bombers detonated bombs on packed Moscow metro trains.

Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theater to end another hostage taking.

Putin, as prime minister, launched a 1999 campaign to crush a separatist government in the Muslim southern region of Chechnya, and as president continued a hard line in suppressing rebellion.