Russia's President Vladimir Putin has said that he had signed a decree on partial mobilization beginning on Wednesday to bolster forces in Ukraine.
Putin made the announcement in a televised address to the nation on the day.
He accused Western countries of trying to weaken, divide and ultimately destroy Russia.
The president reiterated his intention to continue the military operation in Ukraine. He said the decision to conduct the preemptive operation was necessary and the only choice available, and that its aim is to "liberate" the eastern Ukrainian region of Donbas.
He said the mobilization targets only people in the military reserve, with priority on those who have a military specialty and experience.
Putin also referred to planned referendums on joining Russia, which were announced by pro-Russian separatists in four Ukrainian regions. They said the votes will be held between September 23 through 27.
Putin pledged to do everything possible to ensure safe conditions in which to hold the referendums.
Russia's defense minister Sergei Shoigu said on state-run television that 5,937 Russian soldiers have died in the operation in Ukraine.
He said he expects 300,000 people in the reserve to be mobilized under the new decree.