Russia's defence minister said 300,000 reservists will be mobilised.
President Vladimir Putin has ordered Russia's first military mobilisation since World War II.
He also warned the West that if it continued what he called its "nuclear blackmail" that Moscow would respond with the might of all its vast arsenal.
Russia's defence minister said 300,000 reservists will be mobilised.
According to ABC News, Putin in a televised address, said his aim was to "liberate" east Ukraine's Donbas region, and that most people in the region did not want to return to what he called the "yoke" of Ukraine.
He said he was defending Russian territories and that the West wanted to destroy the country.
"We are talking about partial mobilisation, that is, only citizens who are currently in the reserve will be subject to conscription, and above all, those who served in the armed forces have a certain military specialty and relevant experience," Putin said.
He also said the decision to partially mobilise was "fully adequate to the threats we face, namely to protect our homeland, its sovereignty and territorial integrity, to ensure the security of our people and people in the liberated territories.”
“If the territorial integrity of our country is threatened, we use all available means to protect our people- this is not a bluff. “
Putin accused Western leaders of engaging in nuclear blackmail, vowing to respond to any threats against Russia.
"To those who allow themselves such statements regarding Russia, I want to remind you that our country also has various means of destruction, and for separate components and more modern than those of NATO countries," he said.
"When the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal."
Russia already considers Luhansk and Donetsk, which together make up the Donbas region Moscow partially occupied in 2014, to be independent states.
Ukraine and the West consider all parts of Ukraine held by Russian forces to be illegally occupied.
Russia now holds about 60 per cent of Donetsk and had captured nearly all of Luhansk by July after slow advances during months of intense fighting.
Those gains are now under threat after Russian forces were driven from neighbouring Kharkiv province this month, losing control of their main supply lines for much of the Donetsk and Luhansk front lines.
Ukrainian presidential adviser Mykhailo Podolyak told Reuters that Russia's mobilisation was a predictable step that would prove extremely unpopular and underscore that the war was not going according to Moscow's plan.
Podolyak said Putin was trying to shift the blame for starting an "unprovoked war" and Russia's worsening economic situation onto the West.
Putin's address to the nation comes a day after Russian-controlled regions in eastern and southern Ukraine announced plans to hold votes on becoming integral parts of Russia.
The referendums, which have been expected to take since the first months of the war, will start Friday in the Luhansk, Kherson and partly Russian-controlled Zaporizhzhia and Donetsk region.