Adds details on other recent deals and context throughout

By Marek Strzelecki and Anna Hirtenstein

WARSAW/LONDON, Nov 5 (Reuters) - Poland is working on a deal to import liquefied natural gas $(LNG)$ from the U.S. to supply Ukraine and Slovakia, an agreement that would further tighten the European Union’s ties to American energy, according to two sources familiar with the negotiations.

Officials expect to announce a joint declaration to boost imports after a meeting of the parties at a transatlantic energy conference in Athens later this week, one of the sources said.

"After that, discussions would follow on terms for supplies to Slovakia," one of the sources told Reuters. Potential volumes to be shipped south via Poland could be as much as 4 to 5 billion cubic meters of gas per year, about the same as Slovakia's annual gas consumption, the sources said.

This is expected to be the latest in a series of energy deals struck between European and U.S. government officials and companies on the back of a push from Washington to boost exports of American gas and nuclear technology.

The U.S. Department of Energy did not immediately respond to a request for comment. Poland's energy ministry declined to comment.

"We hadn’t fully seized this historic opening that we have to get Europe off Russian energy and onto American, but now this administration is going full throttle and there’s a real shift,” said a U.S. official. “This could seal the deal, we could see a seismic change in how Europe gets its energy.”

The EU put forward new plans to end its purchases of Russian oil and gas last month, with a fresh package of sanctions that bans Russian LNG imports by 2027.

Some EU member states still buy Russian energy, while also supporting Ukraine in the war. Slovakia and Hungary try to balance being allies of Washington and importing Russian energy, which has drawn criticism from U.S. President Donald Trump.

U.S. Energy Secretary Chris Wright in September privately gave European ministers "a very strong message about America’s reliability as an energy partner and America’s interest in doing more with Europe," a former diplomat said.

The same week, U.S. Interior Secretary Doug Burgum met Italy’s energy minister, pledging to deepen ties through increased U.S. LNG sales. Two days later, Italian utility Edison inked a 15-year supply deal for the liquefied gas.

Wright met Polish and Slovak officials in Vienna the following week. Slovakia signed an agreement with the U.S. for a new nuclear power plant on October 7.

The EU’s use of LNG has climbed to historic highs this year, with the U.S. currently supplying about 55% of its supply, up from 27% in 2021. Fully replacing all Russian gas imports with American LNG would raise this to over 80%, according to Reuters calculations.

(Reporting by Marek Strzelecki in Warsaw and Anna Hirtenstein in London; additional reporting by Timothy Gardner; editing by Mark Heinrich, Alexandra Hudson)

((Anna.Hirtenstein@thomsonreuters.com;))