After a warm welcome in the Middle East and a "fantastic" visit with the pope, US President Donald Trump walks on shakier ground on Thursday when European Union and NATO leaders will press him on defense, trade, and environmental concerns.

The Republican president, midway through his first foreign trip since taking office, has basked in the glow of favorable receptions in Riyadh and Jerusalem, where leaders lauded his harsh words for Iran.

Praise may be in shorter supply in Brussels. Trump questioned the relevance of the NATO military alliance as a presidential candidate, and is considering pulling the United States out of the Paris agreement on climate change – a huge concern in Europe. The EU was also a party to the Iran nuclear agreement, which Trump has criticized sharply.

US President Donald Trump (C) takes part in a meeting with US Secretary of State Rex Tillerson (L) and officials on May 24, 2017, in Brussels. /VCG Photo

"We expect him to recommit to NATO's founding rule that an attack against one ally is an attack against all," said a senior European diplomat at the North Atlantic Treaty Organization.

Trump will also meet Europe's chief executive Jean-Claude Juncker and European Council President Donald Tusk who chairs EU summits.  

He will then go to NATO's new, billion-dollar headquarters where he will unveil a memorial to the Sept. 11, 2001, attacks on New York and Washington.

There Trump, in his only scheduled public remarks before a dinner with NATO leaders, is expected to pledge his full support to the alliance he once called "obsolete" because he said it was not doing enough to stop terrorism.

European Commission President Jean-Claude Juncker chairs a meeting of the EU executive body in Brussels, Belgium May 10, 2017. /VCG Photo

NATO hopes to impress Trump with military bands, allied jets flying overhead and a walk through the glass-and-steel headquarters, which replaces a leaking, 1960s prefab structure.

US Secretary of State Rex Tillerson said Trump would press NATO leaders hard to spend more on defence and take on more of the burden of paying for the alliance, a message Trump has reiterated repeatedly before and after entering the White House.

Trump wants NATO to join the battle against Islamic State, Tillerson told reporters on Air Force One.

NATO ambassadors agreed on Wednesday for the Western military alliance to join the US-led, 68-nation coalition against Islamic State in Syria and Iraq, paving the way for a formal endorsement by NATO leaders.

(Source: Reuters)