Sanchez said Sunday that Spain would not need to hit the headline figure of five percent of GDP demanded by US President Donald Trump, setting up a potential clash at a two-day NATO summit starting on Tuesday in The Hague.

NATO diplomats have disputed Madrid's view that it was granted an exception. But Sanchez backed up his claim by posting on X a letter from NATO chief Mark Rutte dated June 22.

"I can hereby confirm that the agreement at the upcoming NATO Summit will give Spain the flexibility to determine its own sovereign path for reaching the Capability Target goal and the annual resources necessary as a share of GDP, and to submit its own annual plans," the text read.

"In addition, the trajectory and balance of spending under this plan will be reviewed in 2029".

Under a deal greenlit by NATO's 32 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.

A NATO diplomat speaking on condition of anonymity Monday said there was "no opt-out for any ally", Spain included.

Rutte's letter only underscored that NATO members have the right to determine how they'll deliver on their pledge, the diplomat told AFP.

"It affirms that Allies chart their own course for making good on their commitments," the diplomat added.

Sanchez posted Rutte's letter online on Sunday in response to a post by centre-right opposition leader Alberto Nunez Feijoo, who had accused him of "propaganda".

"Here you go, Alberto. Ask someone to translate it for you. See if that clears up your doubts", Sanchez wrote.

According to the centre-left leader, each NATO member needed to invest different sums to meet their military capability targets, with the military estimating that 2.1 percent would suffice for Spain.

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

The country is only set to hit the alliance's current target of two percent this year after a 10-billion-euro ($11.5 billion) injection.

Sanchez is facing a difficult balancing act of aligning with NATO allies and cajoling his junior coalition partner, the far-left alliance Sumar, which is hostile to increasing military spending.