NATO leaders were set to sign up on Wednesday to a big increase in defence spending at a short summit tailor-made for U.S. President Donald Trump, who struck a reassuring tone on his commitment to protecting fellow members of the alliance.

The summit in The Hague is expected to endorse a higher defence spending goal of five per cent of gross domestic product — a response to a demand by Trump and to Europeans' fears that Russia poses a growing threat to their security following the 2022 invasion of Ukraine.

"I've been asking them to go up to five per cent for a number of years," Trump said Wednesday morning as he met with Mark Rutte, the alliance's secretary general. "I think that's going to be very big news."

Speaking to media before the summit opened, Trump also played down concerns over his commitment to mutual defence among allies as set out by Article 5 of NATO's charter, saying: "We're with them all the way."

The debate has been fuelled by Trump's own comments en route to the summit on Tuesday. Speaking to reporters aboard Air Force One, he had said there were "numerous definitions" of the clause.

WATCH l Carney on Canadian defence spending plans: 

Carney tells CNN that new 5% NATO benchmark would cost Canada $150B a year

In an interview with CNN, Prime Minister Mark Carney acknowledged that the new spending target is 'a lot of money,' but that Canada can already account for some of it. NATO allies will debate spending 3.5 per cent of GDP on defence and 1.5 per cent on defence-related investments, and Carney said 1.5 per cent figure covers things Canada is already doing such as infrastructure spending.

'There is no alternative': NATO's Rutte on increase

The new spending target — to be achieved over the next 10 years — is a jump worth hundreds of billions of dollars a year from the current goal of two per cent of GDP, although it will be measured differently.

Countries would spend 3.5 per cent of GDP on core defence — such as troops and weapons — and 1.5 per cent on broader defence-related measures such as cyber security, protecting pipelines and adapting roads and bridges to handle heavy military vehicles.

Rutte acknowledged that it was not easy for European countries and Canada to find the extra money but said it was vital to do so.

"There is absolute conviction with my colleagues at the table that, given this threat from the Russians, given the international security situation, there is no alternative," he told reporters.

Hungarian Prime Minister Viktor Orban diluted the display of unity when he told reporters that NATO had no business in Ukraine and that Russia was not strong enough to represent a real threat to NATO.

All NATO members have backed a statement enshrining the target, although Spain declared it does not need to meet the goal and can meet its commitments by spending much less.

U.S. concerns about alliance spending

While the issue has become a hobby horse the past decade for Trump, U.S. predecessors Barack Obama and George W. Bush also urged alliance members to bolster their domestic defence spending.

Trump has often characterized countries under NATO benchmarks as being "delinquent," but the previous guidance referred to an individual country's defence commitment, not their contribution to a joint NATO defence fund.

As well, some analysts have said the flat metric can provide an incomplete picture of a country's military capabilities, its readiness and deployability.

"It does not assess whether a country spends its limited resources wisely," the think-tank Carnegie Europe once said in a report. 

WATCH l Jennie Carignan, chief of defence staff, on Canada's ability to boost spending: 

Can Canada spend $150B a year on defence?

NATO members will soon decide whether to raise the GDP defence spending target to five per cent — which Prime Minister Mark Carney says would cost Canada up to $150 billion per year. This comes as a shaky ceasefire takes place between Israel and Iran. Chief of the Defence Staff Jennie Carignan gives an update on troops in the region, and discusses the possibility of growing the armed forces with increased defence spending.

Trump was scheduled to meet Ukrainian President Volodomyr Zelenskyy on Wednesday. The pair had a contentious, truncated Oval Office meeting in February in Washington, though they subsequently spoke briefly while at Pope Francis's funeral in April.

In his first term as American president, Trump suggested to Zelenskyy in a phone call that the Ukrainian leader should co-operate in efforts to discredit political rival Joe Biden. U.S. aid to Ukraine was delayed, Democrats impeached Trump for what they said was a quid pro quo, though Trump was eventually acquitted in the Senate.

Attempts by the second Trump administration to mediate a ceasefire in the Russia-Ukraine war have so far been unsuccessful.

WATCH l Trump blasts leaks on intelligence report: 

Trump pushes back on leaked report saying U.S. strikes did not destroy Iran nuclear sites

U.S. President Donald Trump is doubling down on his assertion that Iran's nuclear program was destroyed by U.S. strikes. 'I believe it was total obliteration,' he said. His comments followed reports on Tuesday that the U.S. Defense Intelligence Agency had assessed that the strikes had set back Iran's nuclear program by just a few months.

Trump team angered by reports on Iran strike

While in Europe, Trump said that the damage to Iranian nuclear sites from American missile strikes over the weekend was severe, even as he acknowledged that the available intelligence on the matter was inconclusive.

His comments followed reports by Reuters and other media outlets on Tuesday revealing that the U.S. Defense Intelligence Agency had assessed that the strikes had set back Iran's nuclear program by just a few months, despite administration officials saying the program had been obliterated.

"The intelligence says, 'We don't know, it could have been very severe.' That's what the intelligence says. So I guess that's correct, but I think we can take the 'we don't know'. It was very severe. It was obliteration," Trump added.

Later, during the same round of comments, Trump argued that Iran's nuclear deal had been set back "basically decades, because I don't think they'll ever do it again."

Secretary of State Marco Rubio and Secretary of Defense Pete Hegseth also cast doubt on the reliability of the DIA assessment. Rubio said the U.S. was opening an investigation into the leak of the DIA report. He also suggested the report's contents had been misrepresented in the media.