Why Moscow's war machine is like a 'sponge'
Russian forces are unlikely to run out of soldiers, Michael Clarke says, because "it is recruiting people from all over the world and because it's paying them large amounts of money".
Nearly one million Russian troops have been killed or wounded in the war, while around 400,000 Ukrainian troops have also been killed or wounded, according to the Washington-based Centre for Strategic and International Studies.
Moscow's war machine is paying soldiers from the east of Russia a lot to fight in Ukraine, as well as soldiers from foreign countries, Clarke says.
There are also foreign workers from Asia and Africa working in Russia's arms factories.
"It's a big war machine, but it's like a sponge: you can punch into it and it keeps resuming its shape, but the sponge itself finds it difficult to create a big offensive, it just keeps spreading out," he says.
"It's very hard, therefore, for the Ukrainians to stop it unless they have a lot more modern equipment and some different tactics."
It will strain their economy, Clarke says, adding the war has "bent their economy out of shape already".
He says that is one reason Vladimir Putin is "going for as much as he can this year".
He also warns the quality of the army could go down as Russia keeps bringing in more foreign nationals into their armies who have not been properly trained.
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