Putin: 'Thousands' from former Soviet bloc fighting with IS
Vladimir Putin has said that 5,000 to 7,000 people from Russia and other former Soviet states are fighting for so-called Islamic State (IS) in Syria.
The Russian leader told a regional forum that the IS fighters would pose a serious risk if they returned home.
Mr Putin also warned on Friday that violence in Afghanistan could spread to Central Asia.
The situation in Afghanistan was "close to critical" and Central Asian states should be "ready to react", he said.
"Terrorists of different stripes are gaining more influence and do not hide their plans for further expansion," Mr Putin told the Commonwealth of Independent States (CIS) meeting in Kazakhstan.
Leaders at the summit of former Soviet Union states have agreed to create a joint task force to defend their bloc in the event of a crisis.
Tajikistan, which has the longest border with Afghanistan out of all the former Soviet republics, is considered to the most at risk, the BBC's Moscow correspondent Steve Rosenberg says.
Russia has a military base in the country and the agreement could lay the ground for Russian and other troops to be deployed along Tajikistan's border with Afghanistan.
US President Barack Obama announced on Thursday that US troops would stay in Afghanistan beyond 2016.
Mr Obama announced that 5,500 troops would remain in the country when he leaves office in 2017 to help local forces counter a growing Taliban threat.
Mr Putin also claimed that Russia's bombing campaign in aid of Syrian President Bashar al-Assad had killed "hundreds of terrorists".
Mr Putin has dismissed the criticism of his bombing campaign and said on Friday that it would continue "for the period of the Syrian troops' offensive operations against terrorists".
He also said Russia was in a "negotiation process" with regional powers, including Saudi Arabia, and was "making attempts to build co-operation" with the US and Turkey.
Syrian rebels and Western governments say Russia has mainly been hitting non-IS targets. The US has accused Russia of running a "fundamentally flawed" campaign in Syria that risks further escalating the conflict there.