European Council President Donald Tusk on Sunday questioned whether tensions between Poland's rightwing government with Ukraine and the EU is a "Kremlin plan".
"Alarm!" warned Tusk, a former liberal prime minister of Poland, on his personal Twitter account.
"Intense dispute with Ukraine, isolation within the European Union, departure from the rule of law and judicial independence, attack on the non-governmental sector and free media - is this PiS strategy or the Kremlin's plan? Too similar to sleep peacefully," he said in Polish.
The governing conservative Law and Justice (PiS) party is increasingly at odds with Brussels because of its controversial court reforms, large-scale logging in a primeval forest and refusal to welcome migrants.
Relations between PiS and Tusk have been so tense that Poland was the only country to vote against his re-election as EU president in March.
Polish PM Beata Szydlo accused the PiS rival of attacking the country.
"(Donald Tusk as European Council President) has done nothing for Poland," she said on Twitter.
"Today, by using his position to attack the Polish government, he's attacking Poland."
Relations between Warsaw and Kiev had strengthened following the annexation of Crimea by Russia in 2014 because of security concerns.
But ties became strained after the Polish parliament last year reopened a dispute over the World War II massacre of 100,000 Poles by Ukrainian nationalists, officially recognising it as a genocide.
Warsaw also accuses Kiev of preventing the exhumation of Polish victims killed in Ukraine during the war.