The war in Ukraine could end in a stalemate, despite Russia's new offensive in the Donbas region, according to two former British Army generals.
The Russians have launched a new push to take the region in south-eastern Ukraine, as they try to achieve at least one objective in a war that has seen them struggle.
Speaking to the Defence Select Committee, retired general Sir Richard Barrons, said: "Much as we would wish it, there is no clock or script attached to this war in Ukraine that says this is going to be finished cleanly or quickly.
"We should establish in our heads, at least, a stalemate may occur, I would say, by late spring, which no one knows how to unstick."
Russia has moved an additional 11 tactical battlegroups to the east, as well as heavy artillery and helicopter support to boost its chances of taking the industrial region of the Donbas.
Former Chief of the General Staff, General Lord Dannatt said it appeared that the Russian military was learning from mistakes it made earlier in the war.
He said: "The Russians have probably learnt the lesson that if they push forward armoured troops and their infantry against prepared Ukrainian positions, they - the Russians - are likely to experience high levels of casualties, which is what they've already suffered from.
"They're not putting their armour forward in great numbers - certainly not yet - but they are mounting a considerable number of artillery strikes both on the Ukrainian defensive positions and more widely over the last few days across central and western Ukraine.
Watch: Russian forces 'embarrassed' by inability to take Kharkiv, former marine reports
"Military people would probably see what I've seen in that: that this is part of the same battle, if you like - the artillery and missile strikes through the country are part of the deep battle, and the artillery strikes on the Ukrainian defenders are part of the close battle."
He added: "I think this is a much more coordinated thrust, and we'll have to wait and see how successful it is or it isn't."
General Lord Dannatt agreed the war could come to a stalemate, saying: "The Ukrainians may have to accept that they control about 85% of their country but they've actually lost 10 to 15% of it.
"Maybe that's where this situation will go in the deep freeze for quite some time."
The Donbas region, which includes the cities of Luhansk and Donetsk, has been contested by Russian-speaking separatists since Moscow annexed Crimea in 2014.
Ukrainian forces are well dug-in, familiar with the ground, and have received training from British forces as part of Op Orbital.
Mariupol, to the south of the Donbas, looks likely to fall to the Russians in the coming days.
Then only the Donbas stands in the way of Putin's strategic aim of creating a land corridor between Crimea and Russia itself.