Hungary's foreign minister summoned Ukraine's ambassador on Tuesday over what he said were Ukraine's attempts to block Hungary's new long-term gas supply deal with Russia, which was signed on Monday.
Foreign Minister Peter Szijjarto said in a statement that this amounted to a "violation of Hungary's sovereignty".
Hungary accused Ukraine of meddling in its internal affairs on Monday after Kyiv criticised it over the signing of a new 15-year natural gas supply deal with Russia's Gazprom.
"We regard it a violation of our sovereignty that Ukraine wants to block a secure gas supply for Hungary," Szijjarto said in a statement.
Ukraine, which stands to lose money on transit payments, issued a statement on Monday saying Hungary's supply deal was a "purely political, economically unreasonable decision" and was to the detriment of Ukrainian-Hungarian relations.
It also said it would ask the European Commission to assess whether the deal respected European energy law, to which Szijjarto responded on Tuesday by saying that he was "outraged".
Under the deal finalised with Hungary at the end of August, effective from Oct. 1 and signed on Monday, Gazprom will ship 4.5 billion cubic metres of gas to Hungary annually, via two routes: 3.5 billion cubic metres via Serbia and 1 billion cubic metres via Austria.
Relations between Hungary and its neighbour Ukraine have been scarred for years by a dispute over the linguistic rights of some 150,000 ethnic Hungarians living in the western Ukrainian region of Transcarpathia.
Kyiv infuriated Budapest in 2017 with a law restricting the use of minority languages including Hungarian in schools.
Prime Minister Viktor Orban's nationalist government responded by blocking Ukraine's efforts to build closer ties with NATO and the European Union, of which Hungary is a member.