BERLIN — Polish President Bronis law Komorowski said that Russian President Vladimir Putin is trying to build a new Russian empire for Moscow and that the region now had to choose whether it wanted "a Cossack Europe or a democratic one".

"Russia has carried out an invasion in Ukraine," the Polish head of state told German public radio, according to excerpts of an interview to be broadcast later on Saturday.

Komorowski said Putin was quite open about his ambitions to "rebuild the empire". The Cossacks long served Russian czars in military and security roles on the borders of the empire and their brand of Russian Orthodox patriotism is admired by Putin.

The Polish president, whose post is largely ceremonial but does give him a say in foreign policy, is an ally of Prime Minister Donald Tusk from the centrist Civic Platform (PO).

"I hope Germans are sufficiently mindful of what a Soviet empire meant for Europe," Komorowski told Deutschlandradio Kultur and Deutschlandfunk, warning against any reprise of the pre-World War Two "appeasement policy of yielding to Hitler".

"First the challenge was Crimea, now it is about further regions of Ukraine and everyone is asking where it will end," he said, reiterating a call from Poland and the Baltic states in particular for NATO's eastern flank to be reinforced.

NATO member Poland is one of the most outspoken critics of Putin's support for pro-Moscow separatists in eastern Ukraine. This week the rebels opened a new front against government forces, a reversal Kiev blames on the arrival of Russian troops.