* Ukraine threatens to act over Hungary-Russia deal
* Hungary accuses Ukraine of trying to block the agreement
Ukraine and Hungary summoned one anothers' ambassadors on Tuesday over Budapest's signing of a new long-term gas deal with Russia, which Kyiv regards as a threat to its national security.
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 .
Foreign Minister Peter Szijjarto said on Tuesday Ukraine's ambassador was being summoned over what he described as attempts by Ukraine to block the gas supply deal.
"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, had said on Monday 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".
Ukraine's foreign ministry told Reuters on Tuesday it had summoned the Hungarian ambassador in a tit-for-tat move.
"Gas transportation bypassing Ukraine undermines our country's national security and Europe's energy security," the foreign ministry's spokesman said in a text message.
"The Ukrainian side will take decisive measures to protect national interests," it said, without elaborating.
Under the deal signed on Monday, which is effective from Oct. 1, 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.
