Russia's top security chief is raising alarm about Islamic extremists massing on Afghanistan's northern border.

Alexander Bortnikov, chief of the main Russian intelligence agency FSB, said on a visit to Tajikistan on Tuesday that some 5,000 fighters of an Islamic State group affiliate have gathered in areas bordering on former Soviet states in Central Asia. Bortnikov, in comments carried by Russian news agencies, called for tighter border control to prevent a spillover.

The IS affiliate emerged in 2014 and refers to itself as the Khorasan Province, an ancient term for an area that includes parts of Afghanistan, Iran and Central Asia.

Russia has been expressing this concern for several years. Some experts say the Kremlin is exaggerating the number extremists to justify Russia's outreach to the Taliban.