Ukraine called Monday on Warsaw to "punish" Polish farmers who forcibly pried open Ukrainian trucks crossing into the EU, spilling mounds of grain onto roads near their border.
The farmers began protesting at the border last week, blocking three crossing points in anger at what they see as cheap Ukrainian grain imports.
The fresh protests mark another blow to relations between Ukraine and Poland, which had only weeks before managed to quell a similar two-month blockade by Polish truckers.
Ukraine's agriculture ministry said it strongly condemned the "deliberate destruction of Ukrainian grain by Polish protesters."
"We expect the culprits to be quickly identified and punished," the ministry said.
Poland said it had opened a probe into "breaking customs security and destroying property" after the incident.
The spilt grain will "probably not be suitable for further use," spokeswoman of the District Prosecutor's Office in Lublin, Agnieszka Kepka, told AFP.
Ukraine, once dubbed "Europe's bread basket", has seen its agriculture sector turned upside down by Russia's invasion, with many of its Black Sea export hubs blocked and farmland rendered unusable by warfare.
"For two years since the full-scale invasion, Ukrainian farmers have been working under constant enemy fire and suffering huge losses," Kyiv's agricultural ministry said
"They obtain this grain with great difficulty and sometimes at the cost of their lives," it said.
The protests represent another awkward blow to the new Polish government led by Prime Minister Donald Tusk, who has sought to repair ties with Kyiv fractured by the last right-wing government.
Tusk was in Paris for talks with French leader Emmanuel Macron on Monday, where he stressed Poland still stood firmly by Ukraine.
'Watering fields with blood'
In Kyiv, outraged officials demanded that Warsaw publicly condemn the incident, chiding what they see as a worrying increase in anti-Ukrainian sentiment.
"The lack of reaction from the Polish authorities to the destroyed cargo will lead to more xenophobia and political violence," Ukraine's deputy economy minister Taras Kachka said on social media.
Polish police on Monday confirmed they had conducted an inspection at the site and interviewed witnesses.
The evidence gathered "will be sent today to the District Prosecutor's Office in Chelm for a criminal law assessment regarding further proceedings in this case", a spokeswoman for the local police, Ewa Czyz, told AFP.
Many Ukrainian officials said the Polish farmers were acting in the interests of Russia.
"Ukrainians are literally watering with blood the fields upon which this grain is grown," Andriy Sadovyi said on social media.
"Harvesting wheat in a field that has seen war is like working as a sapper," he added, calling those who spilt the grain "pro-Russian provocateurs".
The leader of the now-halted Polish trucker protest, Rafal Mekler, said he was at the border on Monday and defended the protests.
Polish hauliers -- who want the re-introduction of restrictions to enter the EU for their Ukrainian competitors -- have vowed to resume a large-scale border blockade if their demands are not met.
Tusk calls for calm
As Kyiv expressed outrage over the protest, Poland's Tusk promised to find "solutions" amid a wave of farmer protests across Europe.
Tusk returned to power last year after eight years of nationalist rule.
"I will look for common solutions with our French friends on food security," he said ahead of a meeting with Macron, according to Polish news agencies.
"It is hard to find in Europe a more pro-Ukrainian politician than I am," he assured, before adding: "But we also need to take into account the interests of food security of Poland and of Europe."
A day earlier, he had warned that the economic disputes with Ukraine could lead to "sudden anti-Ukrainian sentiment."
Tusk, who met with Ukrainian President Volodymyr Zelensky in Kyiv last month, said on Sunday that "nothing will change" in Poland's intention to help Kyiv fight off the Russian invasion.
But he also vowed to protect Polish farmers and truck drivers from "unfair or inadequate competition."