Madrid said on Sunday it would not need to hit the five percent of GDP demanded by US President Donald Trump, setting up a potential clash at a two-day gathering starting on Tuesday in The Hague.

On Monday Belgian Foreign Minister Maxime Prevot told local media that while Brussels had to show "willingness to get back in line" after years of underspending, the target was beyond its "budgetary reach".

"We may not have done so by making a noisy statement like Spain, but I can assure you that for weeks our diplomats have been working hard to obtain the flexibility mechanisms... that could help to lighten the burden of the Belgian effort," he told RTBF radio. "We're asking for maximum flexibility".

Under a deal greenlit by NATO countries Sunday, allies promise to reach 3.5 percent on core military needs over the next decade, and spend 1.5 percent on a looser category of "defence-related" expenditures such as infrastructure and cybersecurity.

The pledge is seen as key both to satisfying Trump -- who has threatened not to protect allies spending too little -- and helping NATO build up the forces it needs to deter Russia.

Multiple diplomats at NATO said the agreement -- set to be unveiled at the summit -- had gone through with the approval of all 32 nations and that there was no exemption for Madrid.

But Spanish Prime Minister Pedro Sanchez contended he had struck an accord that would see his country keep respecting its commitments "without having to raise our defence spending to five percent of gross domestic product".

The centre-left leader later posted online a letter from NATO chief Mark Rutte confirming the agreement "will give Spain the flexibility to determine its own sovereign path" for reaching the alliance's military capability requirements.

But a NATO diplomat speaking on condition of anonymity Monday said there was "no opt-out".

"It is always the case that Allies have the sovereign right to determine how they'll deliver on their commitments," the diplomat told AFP.

Belgium, like Spain, has been one of the lowest-spending NATO countries on defence in relative terms.

It currently spends 1.3 percent of GDP on defence, well below the current target of two percent that the government has pledged to reach.