Reports say 40 people have been killed in a suicide attack in the breakaway republic of Chechnya.
About 100 others were injured.
Russian officials say two separatist rebels drove a truck laden with explosives into a government building in a Znamenskoye, a town north of the capital, Grozny.
The area is close to the border with Russia and is considered to be under Moscow's control.
The building, which held local police and FSB security services, was levelled by the explosion, as were five other buildings.
It was the most serious incident since a constitutional referendum on 23 March, in which Chechens voted for the republic to remain part of the Russian federation.
The Russian President, Vladimir Putin, said such attacks would not deter his government from pursuing its peace plan for the region.
