US President Donald Trump took a victory lap at Nato's Hague summit on Wednesday, joining leaders in reaffirming the "ironclad" commitment to protect each other after allies agreed to his demand to ramp up defence spending.

The US leader appeared keen to take the plaudits as he secured a key foreign policy win by getting Nato's 32 countries to agree to meet his headline target of five percent of GDP on defence spending.

In a move that will provide reassurance to allies in Europe worried over the threat from Russia, Trump signed off on a final leaders' declaration confirming "our ironclad commitment" to Nato's collective defence pledge that an attack on one is an attack on all.

"It's a great victory for everybody, I think, and we will be equalised," Trump said of the new spending commitment, ahead of the summit's main session.

Diplomats said that behind closed doors Trump insisted there was no greater ally than Washington and urged others to spend some of the new money on US weaponry.

The deal hatched by Nato is a compromise that allows Trump to claim triumph, while in reality providing wiggle room for cash-strapped governments in Europe.

It sees countries promise to dedicate 3.5 percent of GDP to core military spending by 2035, and a further 1.5 to broader security-related areas such as infrastructure.

Entering the meeting, leaders lined up to declare the summit's spending hike as "historic".

Nato Secretary General Mark Rutte, who hosted the summit in his home city of The Hague, said Nato would emerge as a 'stronger, fairer and more lethal' alliance.

He had earlier acknowledged that it was not easy for European countries and Canada to find the extra money, but said it was vital to do so.

"There is absolute conviction with my colleagues at the table that, given this threat from the Russians, given the international security situation, there is no alternative," the former Dutch prime minister told reporters.

The Kremlin on Tuesday accused Nato of being on a path of rampant militarisation. (AFP/Reuters)