Russia's security service says it has arrested eight people following an explosion over the weekend on a key bridge that links the country to Crimea.
The FSB believes it was an attack organised by Ukraine's defence ministry.
Five Russians and three citizens of Ukraine and Armenia have been detained.
Meanwhile, after warning about the risks of "Armageddon" last week, the US President now says he doesn't believe the Russian leader will use nuclear weapons.
Speaking to CNN's Jake Tapper, Joe Biden gave his latest assessment of President Putin's objectives for the war:
"I think he is a rational actor, miscalculated significantly. He talked about the whole idea of needing to be the leader of Russia and uniting all, I think he will be welcomed with open arms."
