The head of Russia's Federal Security Service (FSB) says that militants from the extremist group Islamic State (IS) have been amassing in northern Afghanistan, near its borders with former Soviet republics in Central Asia.
FSB chief Aleksandr Bortnikov spoke on May 21 during a visit to Tajikistan, which shares a long border with Afghanistan.
Moscow has been expressing concern about an IS spillover into Central Asia for years.
Security experts have said that Russia has exaggerated the number of militants in order to justify its outreach to the Afghan Taliban and to suggest to Central Asian governments that they need support from Russia to defend themselves.
The U.S. military estimates that there are about 2,000 IS fighters in Afghanistan.
Bortnikov said he was concerned about what he called the "redeployment" of around 5,000 IS fighters "to the northern provinces of Afghanistan."
Bortnikov called for tighter border control to prevent a spillover.
In recent years, Russia has emerged as a power broker in Afghanistan, where the Soviet Union fought a disastrous war from 1979 to 1989.
In November, Russia hosted peace talks between the Taliban and Afghan political figures.
U.S. peace envoy Zalmay Khalilzad welcomed the talks, although some U.S. officials have claimed that Moscow was promoting itself as a power broker to challenge the U.S.-backed peace process with the Taliban in Qatar.
IS has been active in Afghanistan since 2015, fighting the Taliban as well as Afghan and U.S. forces.