A diplomatic conflict between Hungary and Ukraine deepened on Tuesday when the two countries summoned each other's ambassadors over Budapest's decision to sign a long-term contract to purchase Russian gas. Ukraine considers the deal a blow to its economic and national security interests.

The 15-year deal, signed by Hungary's Foreign Minister Peter Szijjarto and Russian gas monopoly Gazprom, involves 4.5 billion cubic meters of natural gas per year and will account for half of the country's gas consumption. This will guarantee supplies through the newly built Russian undersea gas pipeline Turk-Stream via Serbia and Austria, cutting off Ukraine as a transit country.

Under the agreement, Ukraine will lose millions in transit fees and Kiev said it will ask the European Commission to assess whether the agreement respects European Union energy legislation.

"Today, the reality is that Hungary's energy needs can be secured in the safest way by a long-term contract with Gazprom," Szijjarto said.

In a Facebook post on Tuesday, Szijjarto said he was "deeply outraged" over what he called Ukraine's attempts to block the energy deal.

"The Ukrainians have nothing to do with what we agree on and with whom. We consider it a serious violation of our sovereignty and national security interests that they want to prevent the secure supply of gas to our country, the heating of Hungarian people's homes and the operation of industry," Szijjarto wrote.

Hard blow to ties

Ukrainian Foreign Ministry spokesman Oleg Nikolenko said later on Tuesday that his ministry has also summoned the Hungarian ambassador to brief him on Kiev's stance regarding the new gas deal, adding that it was an appropriate response to Budapest's steps.

Nikolenko explained that gas transportation routes bypassing Ukraine, in Kiev's opinion, undermined its national security and also Europe's energy security.

"The latest agreement between Hungary and Gazprom deals a hard blow to Ukrainian-Hungarian relations. The Ukrainian side will take decisive steps to protect its national interests," Nikolenko said.

According to a Reuters report, Ukraine's energy minister was expected to meet his EU counterpart over the issue, while Russia had warned Ukraine not to interfere in the deal.

Ukraine had earlier said the deal was a "purely political, economically unreasonable decision" and that it would ask the European Commission to assess whether it respected European energy law.

The EU's executive commission said members were free to enter into bilateral gas agreements, but should inform it within three months if the contracted supply exceeds 28 percent of annual consumption. It noted that the deal signed by Hungary, an EU member, exceeded that level.