But with countries across the alliance scrambling to boost budgets, could a deal for a new target be reached by NATO's June summit that allows Trump to claim a win?

- What Washington wants? -

Trump, like other US presidents, has long accused European allies of taking US protection for granted and underspending on defence.

But he has taken a tougher line and threatened not to defend countries who he thinks are scrimping.

US Secretary of State Marco Rubio told NATO counterparts on Thursday they must agree a "realistic pathway" to five percent, and that means Washington would also have to spend more too.

Rubio conceded Europe could not be expected to make the hikes "in one year or two".

- What's the current level? -

Faced with Russia's war on Ukraine, NATO allies have already ramped up spending in recent years.

As Trump stokes fears over US reliability, more plans for further increases are being announced.

"This is probably the biggest increase in defence spending here on the European side of NATO since the end of the Cold War. But we still need more," NATO chief Mark Rutte said.

Rutte says in order to fulfil plans to counter Russia then countries have to spend well above three percent.

Multiple diplomats said NATO's internal estimates put the figure at between 3.5 and 3.7 percent.

Those feeling most threatened by Russia are already well ahead, with Poland and the Baltic States vowing to hit five percent soon.

Germany has paved the way for a major splurge and the EU has announced measures to bolster spending.

But some countries -- such as Spain, Italy and Canada -- are still well below NATO's current two percent target.

The United States -- by far NATO's largest spender in dollar terms -- last year spent slightly less than 3.4 percent of GDP on defence.

- What would five percent take? -

Reaching five percent would be a massive undertaking for countries already struggling with financial strains.

It would mean an increase of over $1.1 trillion a year across NATO, according to the Peterson Institute for International Economics.

For the United States that would mean adding over $400 billion.

"For some countries like Poland, and even Greece, this is an attainable goal given their threat perceptions," said Ian Lesser from the German Marshall Fund think tank.

"For others it is surely out of reach given the mood in society and other spending needs."

One way to pad the figures could be to expand the definition of what counts for military spending to include things like border security, roads and cyber security.

But Rutte has pushed back.

Officials say another possible obstacle could be if the trade war being unleashed by Trump hits European finances.

"Five percent in the short term in unfeasible," Belgian Prime Minister Bart De Wever told local media.

"It's a bit complicated to demand things of us after launching a trade war, after we've been humiliated and insulted all the time."

- Compromise on the cards? -

So far most European allies have been non-committal on how far they're willing to go.

"There is agreement that we need to do more and that we will work towards the summit in The Hague on maybe setting a new target," said Norwegian Foreign Minister Espen Barth Eide.

"Five percent is, of course, much more than the US itself spends, and it's a very high ambition, and we are not ready to commit to a number at this time."

Diplomats said Rubio did not lay out any clear timeframe for when Washington expects NATO to hit five percent, giving hope of wriggle room for a compromise deal by the summit.

Discussions are expected to start in earnest at NATO in May and diplomats say they're looking to be creative to help satisfy all.

If the United States doesn't accept a straight agreement to hike spending to somewhere over three percent, then diplomats have suggested staggering any vow.

That could mean setting a more realistic figure in the short term, with the five percent as a longer-term goal.

The hope is that any agreement would allow Trump to claim victory -- and keep him on board both within the alliance and on the Ukraine war.

"It could just be a great success," said one NATO diplomat.