US President Donald Trump has told his Nato allies in Brussels that all members of the alliance must pay their fair share of defence spending.
"Massive amounts of money" were owed, he said, voicing a long-held US concern that others are not paying enough.
But Nato states' contributions are voluntary and a target of spending 2% of GDP on defence is only a guideline.
The alliance later agreed that member-states would report back annually on defence spending to Nato.
Again condemning Monday's bombing in Manchester, Mr Trump said terrorism must be "stopped in its tracks".
He called for a moment of silence in memory of the 22 adults and children killed in the "savage attack".
Before visiting Nato's new headquarters, which was formally opened at Thursday's gathering, Mr Trump met several EU leaders for the first time, including France's new President, Emmanuel Macron.
His first foreign tour as president will end on the Italian island of Sicily at a G7 summit on Friday.
Little reassurance: analysis by Jonathan Marcus, BBC defence and diplomatic correspondent
For those within Nato uncertain about President Trump's commitment to the alliance, his brief visit to open the new headquarters building in Brussels will have provided little reassurance.
The commander-in-chief of the most powerful military in Nato still seems unclear as to how its defence resources are generated.
He spoke again of countries who fall short of the Nato defence spending target as "owing" money from past years - which is not how things work at all.
He made what seemed intended as an amusing aside, noting he had not asked what the new headquarters building had cost. Well, he ought to know because the US has provided its share of the funding!
And there was no explicit re-statement of his administration's commitment to Nato's mutual security guarantees - the bedrock of the alliance. The mere fact that this question is raised at all shows just how uneasy remains the relationship between Mr Trump and the organisation of which his country is the leading member.
Is the US right about Nato spending?
According to Nato's 2016 annual report, only five countries met the 2% defence spending target - the US, the UK, Greece, Poland and Estonia.
The alliance hopes that all 28 member-states will reach this target by 2024.
"This is not fair to the people and taxpayers of the United States, and many of these nations owe massive amounts of money from past years and [from] not paying in those past years," Mr Trump said.
Germany spent 1.2% on defence in that period, but Berlin argues that its spending on development aid also contributes to international security.
Nato chief Jens Stoltenberg announced after Thursday's talks that member-states would begin submitting annual national plans this year in the interests of transparency.
"Trump has been clear on his commitment to Nato," he insisted. "But President Trump has also been clear in the message to all allies that we have to deliver on the pledge we made to increase defence spending."
Do they agree about Russia?
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.
But at Nato HQ, he said: "The Nato of the future must include a great focus on terrorism and immigration, as well as on threats from Russia and on Nato's eastern and southern borders."
There was some concern that Mr Trump had not mentioned Article Five, Nato's commitment to mutual defence in the event of an attack on a member, but an unnamed White House official told Reuters news agency that the president stood united with other Nato leaders.
Earlier, European Council President Donald Tusk said after meeting Mr Trump: "I'm not 100% sure we can say that we have a common position... on Russia although when it comes to the conflict on Ukraine we were on the same line."
What did Trump say about Manchester?
"Terrorism must be stopped in its tracks or the horror you saw in Manchester, and so many other places, will continue forever," he said.
"You have thousands and thousands of people pouring into our various countries and spreading throughout, and in many cases we have no idea who they are. We must be tough, we must be strong and we must be vigilant."
Earlier, he condemned the leaking of details of the investigation to US media.
How is Nato responding to the fight against Islamist militants?
It has agreed to take a bigger role in the campaign against so-called Islamic State (IS) and others, but France and Germany insist the move is mostly symbolic.
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.
Did climate change come up in Brussels?
"Some issues remain open, like climate and trade," Mr Tusk said after he and European Commission President Jean-Claude Juncker met Mr Trump.
EU leaders are concerned that Mr Trump may abandon a US commitment to reduce greenhouse gases under the UN Paris accord.
Are there signs of any movement on trade?
Mr Juncker said that at his meeting with Mr Trump, both sides had emphasised "that we should have free but fair competition".
"There's going to be a Commission delegation and a Trump delegation in the next few weeks to come together on trade matters because we felt there was too much divergence, too much divergence in our analysis and our measures."
How did it go with Macron?
The atmosphere between the US conservative and the French centrist appeared strained when they met at the US embassy in Brussels.
The two leaders clasped each other's hands, leaning in towards each other slightly. Mr Trump started to pull away, but Mr Macron held on tighter and refused to let him go, the BBC's Tara McKelvey reports.
Another awkward moment came at Nato headquarters when a smiling Mr Trump pushed past the Prime Minister of Montenegro, Duško Markovic.
How did the US First Lady spend her day?
Melania Trump attended the meeting with President Macron and later spent time with his wife Brigitte, visiting the Magritte museum of surreal art in Brussels.