In a mounting row over grain exports from Ukraine, Poland summoned the Ukrainian ambassador to protest remarks at the UN by President Volodymyr Zelensky.
The Ukrainian leader said some countries were only pretending to support his nation as it wages a counteroffensive to retake land taken by Russia. Warsaw took offense at this.
Poland has been one of Ukraine's staunchest supporters after Russia invaded in February 2022, and is one of Kyiv's main weapons suppliers.
Much of the weaponry that the United States and other countries send to Ukraine passes through Poland, which borders Ukraine to the west.
Poland also hosts some one million Ukrainian refugees, who have benefited from various kinds of state aid.
Tensions between Warsaw and Kyiv were sparked by a Polish ban on Ukrainian grain imports, with the goal of protecting its own farmers.
Polish Prime Minister Mateusz Morawiecki was asked Wednesday if his country would continue to back Kyiv, despite this dispute.
"We are no longer transferring weapons to Ukraine, because we are now arming Poland with more modern weapons," Morawiecki said.
EU transit route
Russia's offensive in Ukraine has closed off Black Sea shipping lanes used before the war, resulting in the EU becoming a major transit route and export destination for Ukrainian grain.
In May, the EU agreed to restrict exports to Bulgaria, Hungary, Poland, Romania and Slovakia, seeking to protect farmers there who blamed the shipments for a slump in prices on local markets.
The measures allowed the products to keep transiting through the five countries, but stopped them being sold on the local market.
But on Friday, the European Commission said it was ending the import ban, arguing that "the market distortions in the five member states bordering Ukraine have disappeared".
Poland, Hungary and Slovakia immediately announced they would defy the move.
French Foreign Minister Catherina Colonna, speaking with AFP at the United Nations, said Poland's decision to ban Ukraine's grain was unjustified.
"These tensions are regrettable," Colonna told AFP in an interview on the sidelines of the UN General Assembly.
Citing an EU study, Colonna said that Ukrainian grain imports would not disrupt the market or cripple European farmers.
"There is no market upheaval and there are perhaps internal political considerations on the part of some of our partners which, unfortunately, pushes them to have this position that nothing justifies," the minister said.
The grain issue is particularly sensitive in Poland, where elections take place next month.
The current populist right-wing government of the Law and Justice party has strong support in farming regions.
"We were the first to do a lot for Ukraine and that's why we expect for them to understand our interests," Morawiecki told Polsat News on Wednesday.
"Of course we respect all of their problems, but for us, the interests of our farmers are the most important thing."
Kyiv responded to warnings by Poland, Hungary and Slovakia by announcing that it would lodge a complaint with the World Trade Organisation (WTO).
Morawiecki had warned earlier on Wednesday that he would extend the list of Ukrainian products banned from import if Kyiv were to escalate the grain dispute.
A foreign ministry statement said that "putting pressure on Poland in multilateral forums or sending complaints to international courts are not appropriate methods to resolve differences between our countries."
Kyiv responded by calling on Poland to "leave emotion aside" after it had summoned its ambassador, urging Warsaw to adopt a "constructive" approach in the dispute.