Nato Secretary General Mark Rutte has said there is "no alternative" to the 32 member states spending more on defence, given the threat from Russia and the broader international security situation.

The leaders of the Western defensive alliance have gathered in The Hague, where they are set to commit to spending 5% of national output on defence and related infrastructure. German Chancellor Friedrich Merz has described the meeting as historic.

It is US President Donald Trump's first Nato summit since 2019 and as he travelled he appeared to raise questions about the alliance's mutual defence guarantee under which an attack on one member is seen as attack on all.

"There's numerous definitions of Article Five, you know that right?"

Asked about Trump's remarks, Rutte said on Wednesday that in his view "there is absolute clarity the United States is totally committed to Nato, totally committed to Article Five".

Wednesday's main session is set to last only two and a half hours, with a brief final statement expected to endorse a spending pledge of 3.5% of GDP on defence and a further 1.5% on "defence-related expenditure", although Spain's prime minister, Pedro Sánchez, has described the target as unreasonable.

Western leaders have all had to navigate their relationships with Trump, known for his sometimes unpredictable handling of diplomacy. The two-day Nato summit has already been scaled back, apparently to accommodate his schedule.

Nato leaders gathered on Tuesday night for a group photograph before joining King Willem-Alexander and Queen Maxima of the Netherlands for dinner.

The Nato secretary general earlier told his European colleagues to stop worrying about the US commitment to the Western alliance and focus on investing in defence and supporting Ukraine.

Rutte said Europe and Canada had already committed to more than $35bn (£26bn) in military support for Ukraine this year.

Trump posted a pre-summit message Rutte had sent him, lavishing praise on the US president's handling of Western alliance and the conflict in Iran.

"You are flying into another big success in The Hague this evening. It was not easy but we've got them all signed on to five percent," Rutte wrote, in a message posted by Trump on social media.

He also congratulated Trump on his "decisive action in Iran, that was truly extraordinary and something no one else dared to do. It makes us safer."

Ukraine's Volodymyr Zelensky is due to meet Donald Trump on the sidelines of the Nato summit. The Ukrainian leader had a notoriously difficult meeting with the US president at the White House in February, before a more constructive exchange at Pope Francis's funeral at the Vatican in April.

Hours before Nato leaders arrived in The Hague, at least 20 people were killed in Russian attacks on Ukraine, and German Chancellor Friedrich Merz said every attempt to bring Russia to the negotiating table had so far been unsuccessful.

Missile attacks on the eastern city of Dnipro and the nearby town of Samar killed 17 people and wounded another 160, according to Ukrainian officials. Eighteen children were wounded in the attack on Dnipro, which damaged a kindergarten, schools and a passenger train, they said.

An earlier missile strike on Sumy in the north-east killed three people, including a child.

Although Nato member states are expected to approve a plan to raise the benchmark for defence investment to 5% of GDP by 2035, many of the allies are below that commitment.

The German government backed a budget deal on Tuesday to hit that target by 2029. Some €62.4bn (£53bn) will be spent on defence in 2025, rising to €152.8bn in 2029, partly financed by debt and special funds.

"We're not doing that as a favour to the US and its president," the German chancellor told parliament in Berlin on Tuesday. "We're doing this out of our own view and conviction, because Russia is actively and aggressively endangering the security and freedom of the entire-Euro-Atlantic area."

After the main summit meeting, Merz is due to meet UK Prime Minister Sir Keir Starmer and France's President Emmanuel Macron as well as the leaders of Italy and Poland.

Mark Rutte has spent much of the nine months since becoming Nato Secretary General working to get allies to commit to the 5% target. The figure is more than double Nato members' current 2% guideline and seemed unthinkable

The wording of the summit's final communique is key.

Reaching the 3.5% core defence spending target will still require a significant adjustment for the majority of Nato countries. Out of 32 allies, 27 spend under 3%, with eight hovering well below the 2% threshold set by the alliance in 2014.

On Monday, Prime Minister Keir Starmer pledged that the UK would meet the 5% target by 2035.

He said the UK had to "navigate this era of radical uncertainty with agility, speed and a clear-eyed sense of the national interest". The UK government said it expected to spend 2.6% of GDP on core defence within two years, alongside 1.5% on defence-related areas.

At the bottom of the rung is Spain, whose defence spending is below 1.3%.

Madrid would need to more than double its funding to meet Rutte's new target – something that Socialist Prime Minister Pedro Sánchez has long resisted, arguing it "would not only be unreasonable but also counterproductive".

It would also, crucially, be unpopular at home – not least among his left-wing governing coalition – at a time when Sánchez's government is teetering.

On Sunday Sánchez said Spain had reached a deal that would see it exempted from the target – something Rutte swiftly pushed back on. "Nato is absolutely convinced Spain will have to spend 3.5% to get there," he said on Monday.

Sánchez's suggestion of a lower spending threshold was enough for Belgium and Slovakia to also express interest in an exemption – denting Rutte's hard-won image of a united alliance.

"I can assure you that for weeks our diplomats have been working hard to obtain the flexibility mechanisms," said Belgium's foreign minister Maxime Prévot. Brussels' spending is currently at 1.3% - and Slovakia has also said it reserves the right to decide when to meet the new target.

Despite their comments, all 32 states are expected to sign up to the new pledge.

As Nato leaders and the leaders of more than a dozen partner states made their way to The Hague, train travel from Schiphol Airport near Amsterdam was badly disrupted after cables were damaged by fire.

Security Minister David Van Weel said sabotage could not be ruled out. "It could be an activist group, it could be another country. It could be anything," he told public broadcaster NOS. "The most important thing now is to repair the cables and get the traffic moving again." (BBC)