Kyiv urged Warsaw on Monday to "hold to account" Polish farmers for stopping three trucks at a border crossing between Poland and Ukraine and spilling the Ukrainian grain they were carrying.

Sunday's incident near the Yahodyn-Dorohusk checkpoint, reported by the Polish news agency PAP, was during a wider protest by Polish farmers against European Union agriculture policies, including allowing cheap grain imports from Ukraine.

"The spoiling of Ukrainian grain on the Polish border is unacceptable. Any farmer should know how much hard work it takes to produce grain, especially during wartime," Ukrainian Foreign Minister Dmytro Kuleba wrote on X, formerly known as Twitter.

"For the sake of friendly Ukrainian-Polish relations, the perpetrators of this provocation must be held to account."

Ukrainian trade representative Taras Kachka said on Facebook the spoiled grain was en route to Lithuania and urged the Polish government to ensure the safety of Ukrainian goods.

Regional prosecutor Agnieszka Kepka told Reuters an investigation had been opened into the "removal of a customs seal and destruction of property".

Warsaw has backed Ukraine during Russia's invasion, but relations have been hit by discontent inside Poland against what farmers and hauliers see as a surge in grain and food imports from Ukraine and improper competition.

Polish Deputy Agriculture Minister Michal Kolodziejczak said earlier on Monday that Warsaw plans to start quality checks on all grain shipments from Ukraine.

Representatives of Poland's new pro-European government have said they understand the challenges farmers are facing, and that help for Ukraine in its fight against Russia's invasion cannot come at the expense of Polish farmers. REUTERS