The failure of many European allies to get even close to the NATO target of spending two percent of their national output on defence by 2024 has infuriated Trump, who accuses them of freeloading.

Figures from the International Institute for Strategic Studies (IISS) showed that NATO's 27 European countries fell short of the two percent target by $102 billion in 2018.

The IISS's annual "Military Balance" report said European NATO members would "collectively have had to increase their spending by 38 percent" to hit the two percent target in 2018.

Trump's anger over spending has fuelled concern about his commitment to the transatlantic alliance, culminating in an explosive summit last year where he launched a blistering attack public attack on Berlin in a televised meeting with Chancellor Angela Merkel.

US military spending dwarfs that of the rest of the alliance -- in 2018 Washington spent nearly $650 billion on defence, compared with around $250 billion for all the European NATO members combined, according to the IISS report.

NATO expects seven European countries to meet the two percent target when final figures for 2018 are calculated -- up from just three a year earlier.

But while Germany is making efforts to boost military spending, the sheer size of its economy means it is hard to quickly increase the percentage relative to its hefty gross domestic product (GDP).

To hit the two percent target, analysts say Berlin would need to vastly raise its defence expenditure between 2017 and 2024 -- a problematic undertaking in any country, let alone one with Germany's post-war history of unease about military strength.

The increase in US spending alone from 2017 to 2018 -- around $45 billion -- almost equalled the entire German defence budget, the report noted.