The president has repeatedly railed against European alliance members for not spending enough on their own defence, accusing them of freeloading on US military might.

Apart from the US, six members hit the target of spending two percent of GDP on defence in 2018: Britain, Estonia, Greece, Latvia, Lithuania and Poland.

Though NATO countries have only promised to try to hit two percent by 2024, the failure of many to get even close has infuriated Trump, who reportedly threatened to pull out of the alliance if the European allies did not boost spending immediately.

Germany, Europe's economic powerhouse, has come in for particular criticism and once again it fell well short of the target.

While German defence spending went up from $45 billion to $50 billion, the country's growing economy meant the figure relative to its GDP stayed flat at 1.23 percent.

NATO Secretary General Jens Stoltenberg said in January that Trump's regular outbursts about expenditure had led to allies spending more.

Stoltenberg regularly points out that overall NATO defence spending is rising and that by the end of 2020, allies will have added $100 billion to expenditure since Trump took office in 2016.

US military spending dwarfs that of the rest of the alliance. In 2018 Washington spent nearly $700 billion on defence, compared with just $280 billion for all the European NATO allies combined.

Last month a report by 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 said European NATO members would "collectively have had to increase their spending by 38 percent" to hit the two percent target in 2018.

NATO officials had expected seven European member states to hit the target in 2018 but Romania fell just short on 1.92 percent because its economy grew faster than expected.