Hungary called on the European Union’s executive to launch a probe into Bulgaria’s tax on gas transit, which the government in Budapest says is illegal and threatens energy supplies ahead of the winter.

The European Commission should also ask Bulgaria to suspend the implementation of the levy until the so-called infringement procedure concludes, Hungary’s EU Affairs Minister Janos Boka said in a Facebook post on Friday. If the EU executive doesn’t move, Hungary will file a lawsuit at the European Court of Justice before year-end, he said.

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Hungary has said the tax on Russian gas transit, which Bulgaria announced last month, is a “hostile step” that came without warning. Prime Minister Viktor Orban’s government has vowed to retaliate against the Balkan state, and has said it will work with Serbia, which also relies on the pipeline for natural gas, on a response.