A loud bang heard in a residential building close to the site in St Petersburg where explosives were found earlier in the day was caused by building works.

No one was hurt in the explosion but a vehicle was damaged by falling masonry, news agency RIA reported, citing a source in Russia's emergencies ministry.

Local city administrators told Reuters the bang was caused by renovation works.

Residents were evacuated from a nearby building earlier after police raids searching for accomplices in a terror attack that killed 14 people found an unexploded bomb similar to the one used in the attack.

The discovery raised the possibility a string of bomb attacks was being planned in the city involving a group of plotters.

The device was found inside a flat raided in the early hours in the city’s Tovarichesky Prospekt district.

Local media said materials used in the explosive device matched those used by militants in Syria.

Earlier, the Federal Security Service, the main KGB successor agency, said in a statement carried by Russian news agencies its experts defused a self-made explosive device at the apartment.

“We were told: the house is mined, get out quickly,” one woman, who only gave her name as Tatiana and lives in the building, told Reuters.

Russia President Vladimir Putin lays flowers at scene of blast in St Petersburg

Another resident, who gave his name only as Anatoly, said he had seen police detain four young men occupying an eighth floor flat next door.

“An explosive device found in the flat has been made safe,” local administration head Konstantin Serov said.

“Several suspects have been arrested; they didn't resist and there's now no threat to local people.”

St. Petersburg metro blast Show all 5 1 /5 St. Petersburg metro blast St. Petersburg metro blast An injured person is helped by emergency services outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Emergency services work outside Sennaya Square metro station in St Petersburg Reuters St. Petersburg metro blast An injured person is helped by emergency services outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Emergency services direct pedestrians outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Twitter/ varlamov

Russian security agents have arrested three people suspected of links to the suicide bomber who attacked the St Petersburg metro earlier this week.

The Investigative Committee said the suspects are residents of the former Soviet Central Asia region like the bomber, 22-year-old Akbarzhon Dzhalilov, a native of Kyrgyzstan.

No one has claimed responsibility for Monday's subway bombing, but Russian trains and planes long have been targeted by bombings by Islamist militants.

An unidentified law enforcement official told the Tass news agency that investigators were checking information that Dzhalilov may have trained with IS in Syria.

Russian President Vladimir Putin has said between 5,000 and 7,000 people from Russia and other former Soviet republics were fighting alongside Isis and other militants in Syria.