Russian President Vladimir Putin has signed a decree increasing Russian troop numbers by 15%.
Some 170,000 will be added to the number of serving personnel in stages, bringing the total to 1,320,000.
The defence ministry said the move was a response to an increase in threats, including from the expansion of Nato.
Russia is thought to have sustained heavy casualties in more than a year-and-a-half of fighting in Ukraine, even though it does not release figures.
The Russian defence ministry statement posted on the Kremlin's website said the numbers would be increased gradually through a recruitment drive, and not by mobilisation or changes to conscription.
"The increase in the number of servicemen of the Armed Forces of the Russian Federation is being implemented in stages, based on citizens who express a desire to undergo military service under a contract," it added.
It explained the increase by the "growth of the joint armed forces of [Nato] near Russia's borders" and threats associated with the "special military operation", which is how Russia describes its war in Ukraine.
Nato has recently expanded to include Finland, which has a long border with Russia. Sweden has also applied to join.
The alliance says Ukraine can join "when conditions are met", though it has given no timeframe. Ukraine cannot join Nato while it is still at war with Russia.
Earlier on Thursday Ukrainian President Volodymyr Zelensky called for reinforcements and stronger defences along the front line with Russia, as temperatures in the region fall below freezing.
