If Russian President Vladimir Putin "uses any weapons of mass destruction, (it) will be game-changing in the whole thing... for close alliances", Duda told BBC TV.

In that case, NATO, which Poland joined in 1999, should seriously consider its further moves, the president added.

"Because then it starts to be dangerous, not only for Europe, not only for our part of Europe... but for the whole world," he added, speaking in Polish with an English translation.

Duda said Putin would resort to any means especially as he had already lost this war "politically", while in military terms he is no longer capable of winning it.

Citing expert estimates, Duda said up to five million refugees could leave Ukraine during the war, of whom half will end up in Poland.

His country has so far received almost 1.7 million refugees since Russia invaded Ukraine on February 24.

"More than half of all refugees who have left Ukraine are in Poland. So if there are 5 million, then just imagine that we will get 2.5 million," Duda said.

"It is hard for me to imagine. So we need support here on the ground. We need kind help, financial assistance," he added.

On Sunday, the war got close to the Polish border as 35 people died and 134 were injured in a Russian attack on a military base in Yavoriv in western Ukraine, about 20 kilometres (12 miles) from Poland.