Russia has promised to respond harshly to what it called Poland's illegal seizure of its embassy school in Warsaw, which it said was a flagrant violation of the Vienna Convention on diplomatic relations.

According to Polish state media, police showed up outside the Russian embassy school in Warsaw on Saturday morning to seize the building. 

When asked about the incident, the Polish foreign ministry spokesman said that the building which houses the embassy school belonged to the Polish state. 

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Russia's foreign ministry has hit back at the Polish authorities decision to turn up to the embassy school's grounds with the aim of seizing it.

"We regard this latest hostile act by the Polish authorities as a blatant violation of the 1961 Vienna Convention on Diplomatic Relations and as an encroachment on Russian diplomatic property in Poland," the ministry said.

"Such an insolent step by Warsaw, which goes beyond the framework of civilised inter-state relations, will not remain without a harsh reaction and consequences for the Polish authorities and Polish interests in Russia," it added.

Russia's Investigative Committee said on Saturday that it would give "a legal assessment" of the "seizure," indicating that it will take reciprocal action against Poland.

A spokesperson for the Polish foreign ministry said Russia had a right to protest but added that Poland was acting within the law as the property belonged to the state, claiming that Russia had taken it illegally.

Moscow's ambassador to Poland, Sergei Andreyev, stated that the building was a diplomatic site which Polish authorities had no right to seize.

Relations between Poland and Russia, already fraught, have further soured since the start of the Ukraine conflict, with Warsaw becoming one of Kyiv's staunchest allies and pushing its NATO partners to supply Ukraine with heavy weaponry. 

Earlier in the week, the Russian ambassador said that Polish prosecutors had seized significant amounts of money from the frozen bank accounts of the Russia's embassy and its trade mission.

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