US President Donald Trump has demanded that NATO allies commit to spending five percent of their GDP on defence at their two-day gathering in The Hague, starting on Tuesday.

"The defence investment plan that allies will agree in The Hague introduces a new baseline, five percent of GDP to be invested in defence," Rutte said.

"This is a quantum leap that is ambitious, historic and fundamental to securing our future."

The focus at the summit in the Netherlands will be on keeping Trump happy after his return to power sparked fears that he could blow a hole in the seven-decade-old alliance.

NATO's 32 allies have agreed to a compromise deal to dedicate at least 3.5 percent of GDP to core military needs by 2035, and 1.5 to broader security-related items like cybersecurity and infrastructure.

Rutte said that Spain had not been granted an "opt-out" from the pledge, despite Madrid claiming it had agreed it would not have to reach the headline figure of five percent.

"NATO does not have as an alliance opt-outs, side deals, etcetera, because we all have to chip in," Rutte said.

The NATO chief said that the increase in spending would see the alliance boost its air defences five-fold and add thousands more tanks and armoured vehicles to its arsenals.

"Our focus is ensuring that we have all we need to deter and defend against any threat," he said.

"Of course, the most significant and direct threat facing this alliance remains the Russian Federation."

Rutte insisted that the gathering in The Hague would also send a strong signal of support to Ukraine -- despite Trump upending the West's stance towards Russia's war.

Ukraine's President Volodymyr Zelensky will attend a state dinner with the Dutch king but he has largely been sidelined from the summit's main event.

Rutte said that Europe was stepping up already to fill the gap left by Washington pulling back from supporting Ukraine.

He said that Europe and Canada had already pledged 35 billion euros ($40 billion) to help arm Kyiv so far this year.