Poland’s nationalist president Karol Nawrocki announced Friday he was stripping Ukraine’s Volodymyr Zelensky of the country’s top award, escalating a row between the neighbours and allies over over WWII massacres.

Relations between Warsaw and Kyiv have been tense for days over Zelensky’s naming of a Ukrainian army unit after a WWII nationalist insurgent army that took part in massacres against Poles.

Nawrocki’s decision landed days before Poland is due to host the annual Ukraine Recovery Conference in its Baltic city of Gdansk. It is not clear if Zelensky will attend.

Nawrocki revoked Zelensky’s Order of the White Eagle — Poland’s highest award — over the objections of both Kyiv and Poland’s Prime Minister Donald Tusk.

“Historical truth is not, and can never be, a bargaining chip,” he said in a statement announcing the decision.

Nawrocki said Poland had “repeatedly” called on Ukraine to reverse the naming of the unit. “The Ukrainian side’s position has not changed,” he added.

“Therefore, in light of President Volodymyr Zelensky’s consent to name one of the units of the Armed Forces of Ukraine ‘Heroes of the UPA’… I have decided to revoke the Order of the White Eagle from the President of Ukraine,” he said.

Kyiv said the decision was a “strategic mistake” from which “only Moscow stands to gain”.

Ukrainian Foreign Minister Andriy Sybiga said he now planned to return an award he had received from Poland in 2022 after the “unjustified, impulsive and disrespectful” decision.

Poland has been one of Ukraine’s main allies during more than four years of Moscow’s invasion, taking in hundreds of thousands of refugees and serving as a logistics hub for Western aid to Ukraine.

Historical rows between the neighbours have become less frequent since Tusk returned to power, replacing Warsaw’s previous nationalist government, but they have still been a source of friction between the allies.