At least 4,500 Russians have left the country to fight with "terrorists" in the Middle East, North Africa and other regions, the head of the FSB security service, Alexander Bortnikov, said on Tuesday.

Speaking in an interview with government newspaper Rossiiskaya Gazeta, Bortnikov said that "as of today, some 4,500 Russians had been identified to have gone abroad to take part in fighting on the side of terrorists."

Bortnikov said the security service worked to prevent militants from the Middle East, North Africa, Pakistan and Afghanistan from coming to Russia and stopping Russians from going there.

Over the past two years more than 200 people had been prevented from travelling to those regions, Bortnikov said in the interview released Tuesday.

This year, Russia thwarted 23 terror attacks, he added, noting that more than 9,500 people had been charged with terrorism and extremism over the past five years, Bortnikov added.

Among the security service's priorities were identifying terrorist sleeper cells and potential "lone-wolf" attackers, he said.

President Vladimir Putin earlier Tuesday thanked heads of security services of ex-Soviet countries for sharing intelligence with Russia.

He called international terrorism "the most dangerous threat," saying Moscow was carefully tracking the movement of militants from Syria where Russia's airforce has been engaged in a bombing campaign on the side of strongman Bashar al-Assad.

"We understand where all this scum will be drifting," he said.

Over the past 20 years Russia fought two wars with separatists in Chechnya and Russians frequently became targets of suicide bombings and attacks by Islamist militants.

Moscow's military intervention in Syria in September 2015 has also made Russia a priority target for the Islamic State group.

The country has suffered several attacks this year, including a bombing on the Saint Petersburg metro in April that killed 15 people.

The Central Intelligence Agency of the United States helped Russia last week thwart a planned attack on a landmark Orthodox cathedral in Saint Petersburg, with Putin on Sunday thanking US President Donald Trump for the intelligence.

Putin spoke ahead of Wednesday's 100th anniversary of the establishment of the Soviet Union's first secret police, the All-Russian Emergency Commission for Combating Counter-Revolution and Sabotage known as the VChK.