Polish President Andrzej Duda said Sunday the use of chemical weapons by Moscow in its invasion of Ukraine would be "game-changing" and require a rethink of the conflict by NATO.
His warning came as a senior Ukrainian police officer accused Russian forces of launching phosphorous bomb attacks in the eastern region of Lugansk. (see below)
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.
Moscow accused of phosporous gas attacks in Donbas
A senior Ukrainian police officer has accused Russian forces of launching phosphorous bomb attacks in the eastern region of Lugansk.
International law prohibits the use of white phosphorous shells in heavily populated civilian areas, but allows them in open spaces to be used as cover for troops.
Oleksi Biloshytsky, head of police in Popasna, around 100 kilometres (60 miles) west of Lugansk city, said late Saturday that Russian forces had used the chemical weapon in his area.
"It's what the Nazis called a 'flaming onion' and that's what the Russcists (amalgamation of 'Russians' and 'fascists') are dropping on our towns. Indescribable suffering and fires," he wrote on Facebook.
It was not immediately possible to verify the comments.
The Lugansk and Donetsk regions of eastern Ukraine, collectively known as the Donbas, were partially controlled by Moscow-backed separatist rebels before the full-scale Russian invasion of Ukraine on February 24.
Overnight on Saturday, a train evacuating people from the Donbas to the western city of Lviv was shelled, according to Donetsk military commander Pavlo Kirilenko.
One person was killed and another wounded, he said.
Two Orthodox churches sheltering civilians in the Donbas were also hit, the regional authorities said -- the renowned Sviatoguirsk church in the Donetsk region and a church in Severodonetsk, Lugansk.
There were no details of any casualties.
The areas targeted were not within the so-called separatist "republics" of Lugansk and Donetsk declared by the pro-Russian rebels before the start of the war.