SAINT PETERSBURG -- A woman wounded in the Saint Petersburg metro bombing died in hospital Thursday, a local official said, taking the death toll from the attack to 15.

   Deputy regional governor Anna Mityanina said a woman born in 1960 had died from serious injuries sustained in the April 3 blast which ripped through an underground train.

   Another 23 people are still in hospital, four of them in serious condition, she said.

   Authorities say the attack was carried out by 22-year-old suicide bomber Akbarjon Djalilov, a Russian national born in Kyrgyzstan.

   Ten men from Central Asia have been detained in connection with the bombing, including Kyrgyz-born Abror Azimov, whom officials say was one of the organisers.

   Both he and his elder brother Akram -- whom the FSB says was allegedly in contact with global terror groups -- are in custody.

   There has been no claim of responsibility but investigators are looking into possible links to Islamic State jihadists, who have threatened to strike Russia in revenge for its intervention in Syria.

   FSB head Alexander Bortnikov told reporters on Thursday that the agency was close to establishing who ordered the bombing, Russian news agencies reported. (AFP)