His comments came in a message to the Business Forum of a Three Seas Initiative summit being held in Bucharest on Monday and Tuesday.
Summit participants, including Poland's head of state and eight other presidents, were expected to approve a list of some 40 priority economic projects and to agree to set up a network of Chambers of Commerce.
The Three Seas Initiative, launched by Croatia and Poland in 2015, aims to step up cooperation among 12 countries between the Black, Baltic and Adriatic Seas: Austria, Bulgaria, the Czech Republic, Croatia, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, Slovakia and Slovenia.
In a message posted on the website of the US embassy in Romania, Trump said: “Last year, I had the honor of speaking at the Three Seas Initiative Business Forum in Warsaw, Poland.
"During my visit, I reaffirmed the United States’ strong support for the Three Seas Initiative and our shared focus on expanding infrastructure, enhancing business connections, strengthening energy security, and reducing barriers to free, fair, and reciprocal trade in Central and Eastern Europe.”
He added: “The Three Seas Initiative has great potential to deliver on these and other important areas of cooperation. New energy infrastructure is essential to supporting deeper economic integration, opening access to new markets, and diversifying energy sources throughout the region.”
Poland aims to diversify its energy supplies and reduce its dependence on Russia.
During a visit to Warsaw last July, Trump said that America would be the “strongest ally and steadfast partner” of the Three Seas Initiative.
Polish President Andrzej Duda was on Monday set to fly to Bucharest to take part in the Three Seas Initiative summit.