Poland's Foreign Ministry says the country is investigating the burning of a Ukrainian flag during a march by far-right nationalists, an incident that comes amid increased tensions between the Slavic neighbors.

Ministry spokeswoman Joanna Wajda said on Twitter that police are investigating the flag burning, which occurred Friday during an Independence Day march by 75,000 far-right nationalists.

Wajda also said Monday that Ukraine sent a diplomatic note of complaint to Poland over the matter.

The nationalists burned the flag while screaming vulgarities about Stepan Bandera, a fascist Ukrainian leader during World War II whose followers slaughtered many Poles.

Poland and Ukraine are allies, but have seen tensions grow recently as Ukraine has rehabilitated Bandera. A new Polish film about the massacres of Poles by Ukrainians has added to the tensions.