US President Donald Trump listens to NATO Secretary General Jens Stoltenberg's speech during the unveiling ceremony of the Berlin Wall monument, during the NATO summit

U.S. President Donald Trump meets eastern NATO allies in Warsaw on Thursday amid expectations he will reaffirm Washington's commitment to counter threats from Russia after unnerving them in May by failing to endorse the principle of collective defense.

En route to a potentially fractious G-20 summit in Germany, Trump will take part in a gathering of leaders from central Europe, Baltic states and the Balkans, an event convened by Poland and Croatia to boost regional trade and infrastructure.

The White House has said Trump will use the stopover in Warsaw to showcase his commitment to the North Atlantic Treaty Organisation, which he once called "obsolete", a likely effort to patch up relations after the tense alliance summit in May.

Poland's conservative and euroskeptic government, which shares views with Trump on issues such climate change, migration and coal mining, has hailed the U.S. president's visit as a recognition of its role as a leading voice in central Europe.

The west Europeans, critical of Poland's democratic record, will be watchful as to whether Trump, who will give a major policy speech on a Warsaw square, may encourage its government in its defiance of Brussels.

Some west European governments are worried over a deepening divide between east and west within the European Union and some diplomats see Thursday's regional summit as a Polish bid to carve out influence outside EU structures.

Poland also wants to buy liquefied natural gas from U.S. companies to counterbalance Russian gas supplies in the region.

"We are simply an important country in this part of the world," Polish President Andrzej Duda said in an interview with the PAP news agency.

"We are among the biggest countries in Europe, we are a leader of central Europe, and President Trump ... understands this."