US President Donald Trump is in Brussels for what his team are describing as “tough” talks with other members of the Nato military alliance.
Mr Trump is also due to meet European Union officials on Thursday. He has been critical of both Nato and the EU.
After landing in Brussels, Mr Trump met the Belgian king and queen as thousands demonstrated against his presence in Brussels city centre.
Earlier, he was granted a short private audience with the Pope at the Vatican.
Mr Trump has voiced criticism of other Nato countries for spending less on defence than an agreed 2% of national output
Ahead of Thursday’s meetings, US Secretary of State Rex Tillerson told reporters that Mr Trump “really wants to persuade Nato members to step up and fully meet their obligations”.
He added: “I think you can expect the president to be very tough on them and say… ‘We are doing a lot. The American people are doing a lot for your security, for joint security. You need to make sure you’re doing your share for your own security as well.’
“That’s going to be the core of his message to Nato.”
Mr Tillerson also said President Trump, who has been pressed by European allies to support the Paris climate accord, had yet to make a final decision on whether the US will pull out of the agreement.
Reports say the Nato gathering will see France and Germany agree to a US plan for Nato to take a bigger role in the fight against Islamist militants, particularly the Islamic State (IS) group – but the two European nations insist the move is symbolic.
Before going to Nato HQ, where he will deliver a short speech, Mr Trump is due to have a private lunch with new French President Emmanuel Macron after meeting EU officials.
‘Minor diplomatic masterpiece’ – BBC Europe correspondent Kevin Connolly
Donald Trump’s timetable in Brussels is a minor diplomatic masterpiece of its kind, maximising as it does the number of meetings and minimising the amount of public speaking and press scrutiny which will follow them.
In the morning he’ll meet leaders of the EU, among them European Commission President Jean-Claude Juncker, who once complained that two years would be wasted educating the new president about a world he does not know.
In the afternoon he’ll be at the headquarters of Nato, an organisation he once described as “obsolete” and whose European members he’s criticised for not spending enough on their own defence.
It won’t be difficult though to construct an agenda for those meetings which focuses on very broad areas of agreement – like the importance of the fight against terrorism – and if the day proceeds without diplomatic incident, Europe’s leaders will at least be able to reflect on face-to-face meetings with the world’s most talked about political personality.
Earlier on Wednesday, Mr Trump said he was “more determined than ever” to pursue peace in the world after meeting Pope Francis at the Vatican.
The two men had in the past clashed on issues such as migration, climate change and a Mexico-US wall.
The Vatican said afterwards that the two men shared a commitment to “life, and freedom of worship and conscience” and expressed hope that they could collaborate “in service to the people in the fields of healthcare, education and assistance to migrants”.
On international affairs, they had an “exchange of views” that covered the “promotion of peace in the world through political negotiation and interreligious dialogue”, and highlighted the need to protect Christian communities in the Middle East.
Mr Trump, who BBC Europe editor Katya Adler says seemed star-struck, said of the Pope: “He is something, he’s really good. We had a fantastic meeting and we had a fantastic tour, it was really beautiful. We’re liking Italy very much… it was an honour to be with the Pope.”
Later Mr Trump tweeted: “Honor of a lifetime to meet His Holiness Pope Francis. I leave the Vatican more determined than ever to pursue PEACE in our world.”
The two men exchanged gifts. Mr Trump gave the Pope a boxed set of writings by the civil rights leader Martin Luther King.
The Pope gave Mr Trump a signed copy of a message he delivered for World Peace Day, along with some of his writings about the need to protect the environment.
He also presented him with a small sculptured olive tree, telling Mr Trump through an interpreter: “It is my desire that you become an olive tree to construct peace”.
Mr Trump responded by saying: “We can use some peace.” He also said he would read the texts the Pope gave him.
Mr Trump also met Italy’s president and prime minister while in Rome.
He arrived in Europe from Israel and the Palestinian territories, where he vowed to try to achieve peace in the region.
The US leader began his foreign trip with a two-day stop in Saudi Arabia over the weekend, urging Muslim countries to take the lead in combating radicalisation.
Mr Trump will end his tour on the Italian island of Sicily at the G7 summit on Friday.