Russia on Saturday promised it would respond harshly to what it said was Poland’s illegal seizure of its embassy school in Warsaw, an act it called a flagrant violation of the Vienna Convention on diplomatic relations.

Polish state-run news channel TVP Info had earlier reported that police had showed up outside the Russian embassy school on Kieleckiej street in Warsaw on Saturday morning.

When asked about the incident, a Polish foreign ministry spokesman told Reuters the building housing the embassy school belonged to the Polish state.