(Reuters) -Slovakia will not support a new European Union sanctions package on Russia and will demand a delay in the vote until Slovak concerns over gas supplies after 2027 are resolved, Prime Minister Robert Fico said on Thursday.

The European Commission on June 10 proposed a new round of sanctions against Russia for its invasion of Ukraine more than three years ago, targeting Moscow's energy revenues, banks and military industry.

Slovakia and Hungary have opposed the sanctions due to their disagreement with Commission proposals to end Russian energy imports by the end of 2027, which would force the two countries to look for alternatives.

European diplomats have said they expect the issue to be resolved at the summit.

Fico reiterated his position that the new sanctions could lead to supply shortages and price increases as well as up to 20 billion euros ($23.4 billion) in losses from arbitration for breaching a long-term contract with Russian supplier Gazprom.

Fico said he would back the summit's conclusions, but he would still not agree to the sanctions now.

"Tomorrow, Slovakia will not vote on the 18th sanctions package," he told a parliamentary committee on Thursday before he left for the summit. "We consider it to be one package with (the end of imports plan) and until fundamental issues are resolved, we cannot adopt further sanctions."

Fico met European Commission President Ursula von der Leyen on Thursday but it was not known if that changed his position.

Polish European Affairs Minister Adam Szlapka said at the summit he hoped Slovakia and Hungary could be brought on board as they had been in the past.

"As in the previous sanction packages, I am optimistic here, we are working on it," he said. "I hope that it will be possible to close it by the end of the (EU) Polish presidency, and as we know, there are four days left."

Sanctions proposals require unanimity in the bloc for adoption. Hungary has frequently threatened to withhold its approval during discussions on aid for Ukraine as well as on sanctions renewals, which take place every six months.

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(Reporting by Jan Lopatka in Prague, additional reporting by Julia Payne and Lili Bayer in Brussels, editing by Ed Osmond)