* Trump to speak in Warsaw on July 6
* Will attend Three Seas summit with regional leaders
* Poland hopes Trump's visit spurs investment
By Lidia Kelly
WARSAW, July 4 (Reuters) - Poland hopes to woo Donald Trump the businessman with new investment opportunities in eastern Europe when the U.S. president pays a whistle-stop trip to Warsaw on Thursday en route for a G20 summit in neighbouring Germany.
Amid transatlantic tensions over trade, security and climate change, Poland's right-wing, eurosceptic government wants to put the focus firmly on improving business ties between the United States and ex-communist central and eastern Europe.
Trump, a wealthy businessman, will attend a gathering of the 'Three Seas' initiative, launched jointly by Poland and Croatia last year, which aims to bolster trade, energy ties and infrastructure along a north-south axis stretching from the Baltic states down to the Balkans.
The three seas are the Baltic, the Adriatic and Black Sea.
Some European Union officials are sceptical about the project, which they see as an attempt by Poland - at loggerheads with Brussels on a range of issues - to build a leadership role in a region that includes the EU's poorest member states.
Trump, who has also clashed with the EU on a range of issues, is expected to promote U.S. natural gas exports in his address to about 10 leaders attending from across a region that is keen to reduce its heavy reliance on Russian energy.
Poland, the largest economy in the ex-communist region, received its first shipment of U.S. liquefied natural gas (LNG) last month at its Baltic port of Swinoujscie.
The chief executive officer of Poland's state pipeline operator GAZ-System said the Three Seas summit could give a boost to its North-South Gas Corridor project, which aims to diversify gas supplies away from Moscow.
"Political support is extremely important for such projects, it helps with their timely delivery," Tomasz Stepien told Reuters, adding that this could also help Poland tap EU funds if Brussels deems the corridor a priority.
The North-South Gas Corridor, expected to be completed in the next two-three years, will help send gas supplied from the LNG terminal in Swinoujscie and also from a planned Baltic pipeline through Poland, the Czech Republic, Slovakia and Hungary to a new LNG terminal in Croatia.
Until now the region's main trade routes have tended to run from east to west rather than north to south, partly reflecting Germany's economic dominance - but that is starting to change.
PKP Cargo, Poland's largest rail freight firm, has a deal with Croatia's state railway cargo unit HZ Cargo and is planning other link-ups, including with Romanian ports on the Black Sea.
"Increasing transport between the ports of the Adriatic, the Baltic and the Black Sea is one of our priorities," the firm's Chief Executive Officer Maciej Libiszewski said.
"I think that this region is also important for Americans who, if they wish so, will certainly be welcomed here."
Libiszewski pointed to the U.S. manufacturer of railcars, Greenbrier (GBX.N) , which has a facility in Poland's Swidnica.
Poland's ruling party, Law and Justice (PiS), has backed the construction of the 'Via Carpatia' highway, which is intended to run some 7,750-kilometres from the Lithuanian port of Klaipeda on the Baltic to the Greek city of Thessaloniki on the Aegean, passing through most countries of the region along the way.
"The aim ... is to build a communication backbone that will enable an effective and reliable flow of people and goods," Szymon Huptys, the spokesman for Poland's Infrastructure and Construction Ministry, said.
Many fewer people live along the north-south axis than along the region's east-west trade routes. This may create a challenge in convincing investors that any infrastructure project makes sense.
Asked if the Three Seas initiative could gain traction, one western European diplomat said: "We have our doubts."
But the Poles remain undaunted by the challenges, and insist the Three Seas project will help complement moves among older EU member states in western Europe towards greater integration.
"The Three Seas initiative is not an alternative to European integration processes, but is complementary to them and contributes to the renewal of the European project," Deputy Foreign Minister Konrad Szymanski told Reuters.