PARIS - Leaders of France, Germany and Russia were on the phone on Tuesday discussing ways to boost security cooperation to face terrorism threats after an explosion rocked St. Petersburg metro station, claiming at least 14 lives.

French President Francois Hollande, German Chancellor Angela Merkel and Russian President Vladimir Putin "agreed to further improve their cooperation against terrorism, including the immediate exchange of all useful information", the French presidency said in a statement.

Hollande and Merkel reiterated their solidarity with Moscow and expressed sympathy with the Russian authorities and people.

Investigators said they have searched the home of the suspected suicide bomber behind Monday's deadly explosion.

The bomb went off on a train under Russia's second-largest city on Monday, killing 14 people and injuring dozens. Investigators said they suspect a 22-year old Kyrgyz-born Russian citizen, Akbardzhon Dzhalilov, of having detonated the bomb.

St. Petersburg City Hall said several foreign nationals were among those killed and injured, but would not provide details. The foreign ministry of the Central Asian nation of Kazakhstan said one of its citizens was killed in the attack.

Meanwhile, six people have been detained on suspicion of recruiting for the Islamic State and aiding terrorist activity, Russian investigators said on Wednesday.

There is no evidence the detainees, who are citizens of Central Asian states, are in any way linked to Monday's bomb attack in the metro train, Russia's state investigative committee said in a statement.

The detainees are suspected of being active since November 2015 in recruiting other Central Asian immigrants for "terrorist activity" and for IS as well as another banned extremist movement the al-Nusra Front, which has links to al-Qaida.

Extremist literature has been found during a search of the detainees' living quarters, the statement added.