PUTIN does not accept that Ukraine has the right to exist as a separate state, but regards it as part of a 'Greater Russia', a University of York expert says.

And the Russian leader's expansionist plans may not stop with Ukraine, warns historian Dr Shane O'Rourke.

Putin does not accept the legitimacy of any of the states set up by the Bolsheviks in 1917 - not only Ukraine, but also central Asian states such as Uzbekistan and Kazakhstan, all of which were part of the Russian Empire before 1917.

"They will be worried," Dr O'Rourke said.

Putin may even be looking at the Baltic states and Poland, Dr O'Rourke warned - which could ultimately mean the prospect of war with NATO.

"It is not just about Ukraine," Dr O'Rourke said. "The conflict could expand very easily. If it involved the Baltic states or Poland, then that would mean war with NATO.

"He doesn't want that - no-one wants that. But it could spiral, and he seems ready to risk it."

Putin's speech earlier this week as as he unilaterally declared two enclaves in eastern Ukraine - Donetsk and Luhansk- to be independent states was barely rational, Dr O'Rourke said.

But it is clear that he sees himself as a great figure in Russian history - one who is determined to reclaim territory be believes is Russian. "He wants to recreate Greater Russia, with an eastern Europe that is subservient to Moscow."

Short of all-out war, it is difficult to see what NATO can do to stop him, Dr O'Rourke said.

But it is likely that NATO troops will move to Poland and the Baltic - all states which are NATO members.

Ironically, the situation might make NATO stronger, Dr O'Rourke said. Neither Finland nor Sweden are NATO members. "But they will want to join."