The overriding focus of the gathering, which kicks off with dinner hosted by the Dutch king, has been on keeping Trump happy after his return to power sparked fears he could blow a hole in the seven-decade-old alliance.

Allies are keen that Trump, set to touch down in the Netherlands at 1700 GMT, will see the summit as a victory.

But there is still the chance of fireworks as Kyiv said Trump would meet Ukraine's President Volodymyr Zelensky on Wednesday, with organisers hoping the pair will avoid a repeat of their infamous Oval Office dust-up.

Central to the effort to keep Trump on board -- given his threat not to protect lower-spending NATO allies -- will be a pledge to satisfy his insistence that members cough up five percent of their GDP on defence.

To give him a headline victory, NATO's 32 countries have thrashed out a compromise deal to dedicate 3.5 percent to core military needs by 2035, and 1.5 percent to broader security-related areas such as cybersecurity and infrastructure.

NATO says the military build-up is crucial to deter Russia, which officials warn is rapidly rebuilding its forces depleted by the war in Ukraine and could be ready to attack the alliance in five years.

But it is more important for keeping Trump locked into the alliance's mutual defence vow that is founded on US military might.

"There is total commitment by the US president and the US senior leadership to NATO," alliance chief Mark Rutte said.

"However, it comes with an expectation that we will finally deal with this huge pebble in the shoe, this huge irritant, which is that we are not spending enough as Europeans and Canadians."

- 'Radical uncertainty' -

European leaders -- many of whom are struggling to find the money that will be required -- lined up to argue that the threats facing the continent required bold steps.

"We must navigate this era of radical uncertainty with agility," British Prime Minister Keir Starmer said Monday in announcing the UK's commitment to meet the target.

In a joint Financial Times op-ed on the summit's eve, French President Emmanuel Macron and German Chancellor Friedrich Merz said Europe must rearm "not because someone asks us to, but because we are clear-eyed and owe it to our citizens to do so".

European powerhouse Germany announced plans to hit the 3.5-percent figure for core defence needs by 2029 -- six years before the timeline.

For its part, the Kremlin attacked NATO for its "rampant militarisation", with spokesman Dmitry Peskov saying: "This is the reality that surrounds us."

But while the promise of more spending looks like it could win Trump over, deep divisions remain over the approach to Europe's key security issue: Russia's war in Ukraine.

Since storming back to power, Trump has upended the West's approach to the three-year conflict by turning his back on Kyiv and opening the door to closer ties with Moscow.

- Trump-Zelensky meeting? -

Zelensky has been downgraded from the central role he played at recent NATO gatherings and will not attend the main working session.

But Ukraine's presidency said he would discuss with Trump buying a package of weapons made up mainly of air defences.

Zelensky would also push Trump on imposing new sanctions on Russia as Moscow has stalled peace efforts being pressed by Washington, Kyiv said.

The French and German leaders, in a message aimed at Trump, said they would jointly press for a ceasefire in Ukraine and ramped-up pressure on Russia "including through sanctions" while in The Hague.

Rutte insisted the summit would still send the message that support for Kyiv was "unwavering and will persist".

But despite his insistence that Ukraine's bid for membership remains "irreversible", the alliance will avoid any mention of Kyiv's push to join after Trump ruled it out.

Allies have had to push hard just to get Washington to name Moscow as a "threat" in a summit declaration set to be released when NATO leaders hold their main session on Wednesday.