Chilly ties between Poland and Ukraine sparked by a grain dispute have worsen ahead of Poles' parliamentary elections on Sunday.

Marcin Zaborowski, an expert at the Globsec think tank, blames the election campaign to the deterioration of the two allies' friendship.

"The ruling populist Law and Justice (PiS) party is bidding for anti-Ukrainian votes also wanted by the far-right party Confederation," Zaborowski warned.

The rift started in September when the government extended an embargo on Ukrainian grain imports, prompting Kyiv to complain before the World Trade Organization.

Poland — which has been one of Ukraine's biggest arms suppliers — responded by saying it would restrict arms deliveries to Kyiv as it needed to build up its own army.

The weapons are needed by Ukraine's military in its war against Russian occupiers.

The remark was followed by Ukrainian President Volodymyr Zelensky's speech before the United Nations, accusing some European Union countries of "feigning solidarity" and thereby "indirectly supporting Russia."

Poland interpreted his remarks as directed at Warsaw, and summoned Ukraine's ambassador to be reprimanded.

Poland currently hosts around a million Ukrainians displaced by the invasion of their country by Russia.

But according to a poll by the CBOS Institute, support for Ukrainian refugees in Poland fell from 76 percent in July to 65 percent in September — its lowest level since the start of the war.

Another poll by the Ibris Institute showed that around 40 percent of Poles are opposed to extending rights for Ukrainians to access the labor market, healthcare, education and social services.

The fact that the two sides are falling out publicly is also "a big problem for the United States for whom it is essential to maintain unity among allies," Michal Baranowski from the German Marshall Fund said.