Tusk will meet Czech, Hungarian and Slovak premiers Petr Fiala, Viktor Orban and Robert Fico in Prague on Tuesday at a meeting in the so-called Visegrad group.

Support for Ukraine has sown division among the four-nation group of Central European nations -- all NATO and European Union members -- as Orban and Fico have criticised military support for Ukraine.

Orban blocked a 50 billion euro EU aid package for months while Fico has refused to provide military aid to war-ravaged Ukraine, questioning its sovereignty and calling for peace with Russia.

"If you are a NATO and EU member, by definition and by the very nature of this membership you have to support Ukraine in its defence against the Russian invasion," Tusk told reporters.

"If someone fails to understand this, and I will say this openly in Prague tomorrow, they place themselves ... outside our communities, and there is no compromise to be made on this," he told reporters.

Tusk said he would say this to Orban "in the eyes" when they meet in Prague, and added he was "stunned" by Fico's recent statements on Ukraine.

"It won't be a pleasant or simple encounter," Tusk said after meeting Canadian counterpart Justin Trudeau in Warsaw.

Tusk added that the meeting would be a "test" of the Visegrad group.

He said that upon his return home, he would say "openly" if the four-country club still has a reason to exist.

The Visegrad group was founded in 1991, two years after the four countries had ended four decades of Soviet-led communist rule.

Given the historic experience with Moscow-led communist rule, "it is a paradox that Budapest and Bratislava send out these ambiguous signals concerning (President Vladimir) Putin and Russia these days", added Tusk.