The US president has previously criticised Nato members for not meeting spending targets.
US President Donald Trump is in Brussels for what his team is describing as “tough” talks with other members of the Nato military alliance.
Nato has agreed to Mr Trump’s request to join the US-led coalition against so-called Islamic State.
But he may also press members to pay their full financial share.
Mr Trump has already met EU leaders. EU President Donald Tusk said they had agreed on “many areas” but had differences on Russia.
“I’m not 100% sure we can say that we have a common position, a common opinion on Russia, although when it comes to the conflict on Ukraine we were on the same line, ” he said.
Mr Trump has been criticised for his admiration of Russian President Vladimir Putin, and his administration is embroiled in allegations of close ties with Russian interests.
Mr Tusk continued: “However, my main message to President Trump was what gives our co-operation and friendship its deepest meaning are fundamental Western values like freedom, human rights, respect for human dignity.
“The greatest task today is the consolidation of the whole free world around those values, not just interests… This is what we – Europe and America – should be saying.”
Mr Trump is now meeting new French President Emmanuel Macron.
Mr Macron is expected to try to persuade him not to renege on the Paris climate accord.
Mr Trump, who threatened to leave the agreement during his election campaign, is due to make a decision soon.
The Nato gathering will see the alliance agree to a US plan for Nato to take a bigger role in the fight against Islamist militants, particularly jihadist group Islamic State (IS) – but France and Germany insist the move is mostly symbolic.
Nato Secretary General Jens Stoltenberg said: “Today we will decide to expand our support to the coalition with more Awacs [Airborne Warning and Control System] flight time, more information sharing and air-to-air refuelling.
“This will send a strong political message of Nato’s commitment to the fight against terrorism and also improve our co-ordination within the coalition but it does not mean that Nato will engage in combat operations.”
There are concerns that Nato joining the anti-IS coalition could lead to the alliance becoming embroiled in post-conflict Iraq or Libya as it did in Afghanistan, says the BBC’s Jonathan Marcus.
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.”
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.
After landing in Brussels on Wednesday, Mr Trump met the Belgian king and queen while thousands demonstrated against his presence in the centre of the city.
Earlier in the day, Mr Trump said he was “more determined than ever” to pursue peace in the world after meeting Pope Francis at the Vatican.
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.
