Updated

U.S. Energy Secretary Rick Perry on Tuesday said Europe should lessen its dependence on Russian gas and diversify energy sources, speaking at a summit to improve ties between eastern Europe and the U.S. and the European Union.

"There is great security in energy diversity," Perry said in comments made in the Romanian capital, Bucharest. "Energy security is tantamount to national security." He asserted that Europe's dependence on Russian gas has increased from 30 percent to 40 percent in recent years, calling it "a cause for concern."

Perry said countries of Central and Eastern Europe are facing "challenges due to a single (gas) supplier and Russia's aggressive posturing." He cited a nuclear project in the Czech Republic and offshore oil and gas drilling in the Black Sea as alternative energy sources.

Perry was a key speaker at the Three Seas summit Initiative, which aims to boost connectivity in Eastern Europe and reduce the gap between Eastern and Western EU states.

Earlier, Polish Prime Minister Mateusz Morawiecki said his country wants the U.S. to play a key role in Eastern Europe, to strengthen the continent as a whole.

"The United States is standing with us," said. "We have the will and commitment to make the European project stronger."

Romanian President Klaus Iohannis, a centrist with pro-Western views, called the economic presence of the U.S. "a catalyzer for economic cooperation in the area."

Iohannis, who hosted the two-day summit, announced that Germany, "which has multiple economic interests" in the region, would become a partner state in the Three Seas initiative. He also announced that Slovenia would host the next summit in 2019.

European Commission President Jean-Claude Juncker, Perry, German Foreign Minister Heiko Maas, Croatian President Kolinda Grabar-Kitarovic and other heads of state discussed some 40 government-approved projects that aim to boost regional connectivity in transport, energy and the digital fields.

Grabar-Kitarovic called for the economic gap between "the so-called East and West Europe," to be wiped out and for all EU citizens to have "identical and standard" rights.

The Thee Seas initiative is a cooperation of European Union members located between the Adriatic, Baltic and Black seas. Austria is the only member that wasn't formerly communist. The first summit was held in 2016. U.S. President Donald Trump attended the second summit in 2017 in Warsaw, Poland.