Watch: Who's the 'Daddy'? Rutte raises eyebrows with new Trump nickname

He's already the world's most powerful man, but on Wednesday, NATO Secretary General Mark Rutte gave US President Donald Trump a new nickname: the "Daddy."

The two leaders were joshing ahead of the NATO summit when Trump reprised his analogy of warring countries Iran and Israel being like squabbling children.

"They've had a big fight, like two kids in a schoolyard," said Trump.

"You know, they fight like hell. You can't stop them. Let them fight for about two, three minutes, then it's easier to stop," he added.

Rutte quipped: "And then Daddy has to sometimes use strong language."

That was a reference to Trump's expletive-laced outburst as he set off for the NATO summit, visibly angry at the prospect of the Iran-Israel ceasefire breaking down.

The two countries have been "fighting so long and so hard that they don't know what the f**k they're doing, do you understand that?" the president told reporters at the White House.

Trump chuckled at Rutte's reference, admitting: "Everyone said 'Well, you have to use a certain word'."

Known for his combustible outbursts, Trump rarely uses expletives in public, and his F-bomb caught many by surprise.

Rutte has pulled out all the stops to keep Trump onside for what could have been a tricky summit, with the US president unhappy at spending billions defending Europe's NATO allies.

From keeping the summit short to arranging an overnight stay at the king's royal palace, Rutte has lived up to his nickname—the "Trump Whisperer."

Pressed by reporters on whether his praise of - and moniker for - Trump was demeaning coming from an alliance secretary general, Rutte replied: "No, I don't think so."

"I think it's a bit of a question of taste," Rutte said, calling Trump a "good friend" who "deserves all the praise" -- be it for taking "decisive action" against Iran, or forcing NATO allies to ramp up their defence spending.