Trump told a campaign rally in Racine, Wisconsin that allies in NATO "are not paying their fair share" and called the 28-nation alliance "obsolete."
"We are protecting them and they are getting all sorts of military protection and other things and they're ripping off the United States and they're ripping you off. I don't care. I don't want to do that. Either they pay up including for past deficiencies or they have to get out. And if it breaks up NATO it breaks up NATO," Trump said.
The US has pushed for an increase in resources in Eastern Europe as the US-dominated organization continues to put pressure on Russia over Ukraine.
NATO has been gradually building up its forces in Europe for over a decade, but has recently undertaken a huge increase in its presence in the Baltic region and Eastern Europe. The NATO Response Force (NRF) was originally set up following the Prague summit in 2002 as a multinational force made up of land, air, maritime and Special Operations Forces (SOF) that could deploy quickly.
Since then, there has been a steady build-up of forces, particularly around Russia. NATO Allies decided to enhance the NRF in 2014 by creating a "Spearhead Force" within it, known as the Very High Readiness Joint Task Force (VJTF).
In June 2015, the alliance held a huge exercise in the Baltic, with 49 ships, 61 aircraft, one submarine, and a combined amphibious landing force of 700 US Finnish and Swedish troops alongside NATO partners Finland, Georgia and Sweden. Overall, 5,600 troops took part.
British media reported ahead of the meeting that five UK warships and about 530 Royal Navy personnel will be sent as part of what sources described as "NATO's largest military build-up in eastern Europe since the Cold War."
At a press conference before the meeting, NATO Secretary General Jens Stoltenberg said NATO was: "increasing our presence in the eastern part of the alliance with more assurance measures meaning planes, air policing, naval presence in the Baltic Sea and the Black Sea, increased naval presence and also more boots on the ground with more exercises and troops."