Russian President Vladimir Putin on Wednesday authorized the mobilization of citizens with military experience after setbacks in its invasion of Ukraine.
In a televised national address, Putin also said Russia would use "all the means at our disposal" to protect its territory and people, suggesting the possibility of using nuclear weapons.
Ukraine has been enhancing its counteroffensive in the eastern and southern parts of the country, apparently forcing Putin to decide on bolstering the Russian military.
"Our country has various means of destruction," Putin said. "If our territorial integrity is threatened, we will consider all the means at our disposal to protect our country and its people."
Putin has signed a decree on partial mobilization. Russian Defense Minister Sergei Shoigu said in a television interview that around 300,000 citizens who are currently in the reserves could be called up.
Shoigu also said Russian military casualties had reached 5,937 people, in the first official update since March when the defense ministry said 1,351 had died.
Putin's address came a day after Russian-controlled regions in eastern and southern Ukraine announced plans to hold referendums this month on becoming integral parts of Russia.
"We will do our best to ensure that the referendums will be held safely," Putin said.