Poland's foreign ministry on Wednesday denounced pro-Putin and anti-Ukrainian slogans at farmers' protests there, saying they were "possibly" influenced by Russia.
During the protests on Tuesday against competition from imports of cheaper Ukrainian products, farmers in Gorzyczki, southern Poland, unfurled a banner saying "Putin, get Ukraine, Brussels and our government in order".
The picture of the banner was widely circulated on social media, prompting angry reactions from many Ukrainians.
"We believe that this is an attempt to take over the agricultural protest movement by extreme and irresponsible groups, possibly under the influence of Russian agents," the Polish foreign ministry said in a statement.
It called on the protest organisers to "identify and eliminate from their movement the few initiators of such actions".
Polish police said earlier that they had opened an investigation into promoting a fascist or totalitarian regime and inciting hatred.
Poland has seen its ties with Ukraine deteriorating over border blockades staged by farmers, only weeks after Warsaw managed to quell a similar two-month blockade by Polish truckers.
In a separate incident Tuesday, Polish farmers pried open two Ukrainian freight cars at the Medyka border crossing and spilled grain on the tracks.
Ukrainian Infrastructure Minister Oleksandr Kubrakov called it a "political provocation aimed at dividing our nations".
