Ukraine's foreign ministry criticized Croatian President Zoran Milanovic on Tuesday for saying Crimea would never return to Ukrainian control, describing his comment as "unacceptable."

Russia seized the Black Sea peninsula from Ukraine in 2014 and Kyiv has said it will not abandon efforts to regain control of the region.

In remarks on Monday detailing his objection to Zagreb providing military aid to Kyiv, Milanovic said it was "clear that Crimea will never again be part of Ukraine."

"We consider as unacceptable the statements of the president of Croatia, who effectively cast doubt on the territorial integrity of Ukraine," Ukrainian foreign ministry spokesperson Oleg Nikolenko wrote on Facebook.

In the same statement, Nikolenko thanked the Croatian government and people for backing Ukraine's since Russia's invasion in February last year. Croatian Prime Minister Andrej Plenkovic has openly voiced support for Ukraine.

"We highly appreciate and thank the government of Croatia and the Croatian people for their steadfast support of Ukrainians in the fight against Russian aggression," Nikolenko wrote.