The Russian Federal Security Service (FSB) said on Friday it had uncovered a cell of the militant Islamic State (IS) group that was planning imminent terror attacks in the country's second city Saint Petersburg.

“The FSB identified and stopped the activities of a clandestine cell of IS supporters who planned to commit attacks on December 16,” the security service said in a statement carried by Russian news agencies.

According to the FSB, the group planned a suicide attack and “the killing of citizens” in crowded areas of the northern city.

Seven members of the cell were arrested during raids that took place on Wednesday and Thursday. The police confiscated a “large number of explosives used to make homemade bombs, automatic rifles, munitions and extremist literature,” the statement said.

On Tuesday, FSB chief Alexander Bortnikov said Russia was on alert for the possible return of militants from Syria ahead of the World Cup and the presidential election in 2018.

As many as 40,000 fighters travelled from all over the world, including Russia, to join the IS in Syria after the 2014 declaration of its self-styled “caliphate” straddling Syria and Iraq.

In 2015, Russian security services estimated that 2,900 Russian citizens had joined the IS group, as well as “several thousand” Central Asians. Russia has suffered several terrorist assaults in recent years, including a deadly attack in Saint Petersburg that left 14 dead in April this year.