The first wave of Russian reservists, called up due to Vladimir Putin's partial mobilisation, have started to arrive at military bases, according to the Ministry of Defence (MOD).
In an update posted on Twitter, the MOD said "many tens of thousands of call-up papers have already been issued", adding Russia is now facing the "challenge" of training the troops.
"Unlike most Western armies, the Russian military provides low-level, initial training to soldiers within their designated operational units, rather than in dedicated training establishments," the post said.
"Typically, one battalion within each Russian brigade will remain in garrison if the other two deploy and can provide a cadre of instructors to train new recruits or augmentees.
"However, Russia has deployed many of these third battalions to Ukraine.
The update also said "many of the drafted troops will not have had any military experience for some years" and will "deploy to the front line with minimal relevant preparation".
Watch: Russia's mobilisation will just mean 'more dead bodies' for Moscow.
"They are likely to suffer a high attrition rate," the post said.
Ukrainian President Volodymyr Zelensky told CBS the mobilitsation is a sign of weakness.
"They admitted that their army is not able to fight with Ukraine anymore," he said.
The partial mobilisation is also triggering protests in Russia, with new anti-war demonstrations on Sunday.
Russia has one of the biggest reserve forces in the world but how it is made up is very different to others, including those in the West.
It is reliant on former conscripts and veterans, although Russia in recent times has been trying to increase its number of volunteer reservists, similar to the systems used in the West.
Watch: The tactics behind Ukraine's lightning counter-offensive.
The Institute for the Study of War says few are actively prepared for war, with only 10% of reservists undergoing refresher training following their initial service.
Lieutenant Colonel (Retired) Glen Grant, former advisor to the Ukrainian Ministry of Defence, told Forces News that Russia will be bringing in 300,000 "low grade" troops.
"It’s 300,000 people who don't want to go to war, there's not going to be any of them happy about going," he said.
"The ones they've got at the moment, all they're doing is just pushing them over the line, basically to die."
It comes after Liz Truss said the West should not listen to Putin's "bogus threats" when asked how to respond to Russia's partial mobilisation and reference to using nuclear weapons to defend itself.