"We had one laggard, it was Spain," Trump told reporters in the Oval Office. "They have no excuse not to do this, but that's all right. Maybe you should throw them out of NATO frankly."

In June, the 32-nation North Atlantic Treaty Organization agreed to massively boost defense spending over the next decade under pressure from Trump, who at the time threatened to punish Madrid on trade for resisting the new target of five percent of GDP.

Spain's socialist Prime Minister Pedro Sanchez has insisted Madrid would not need to hit the headline figure. Spain has been one of the lowest-spending NATO countries on defense in relative terms.

The US president -- who has repeatedly suggested Washington could withhold protection from European countries unwilling to spend more on defense -- rammed through the commitment to spend five percent of their GDPs on security-related spending in a move seen as key to keeping him engaged with NATO.

That headline figure breaks down as 3.5 percent on core defense spending and 1.5 percent on a looser range of areas such as infrastructure and cyber security.

The new target replaces the alliance's former military spending goal of two percent, first set back in 2014.