Two powerful blasts were heard on Thursday in the Russian city of Belgorod, near the border with Ukraine, two witnesses told Reuters.
Russia has in recent days reported what it says are a series of attacks by Ukrainian forces on Russian regions which border Ukraine, and has warned that such attacks raise a risk of significant escalation.
Ukraine has not directly accepted responsibility but has described the incidents as payback and "karma" for Russia.
(Reporting by Reuters; Writing by Mark Trevelyan; Editing by Jan Harvey)
