"We strongly support NATO; we only ask that all of the NATO members make their full and proper contributions to the NATO alliance, which many of them have not been doing - many of them have not been even close", Trump said, March 2017.
The current situation, he said, "is not fair to the people and taxpayers of the United States". Some nations, though, have failed to meet that threshold.
Most NATO members, meanwhile, spend less than the 2 percent benchmark.
Germany and France had expressed concerns that upgrading NATO's involvement could skew the geographical balance among the existing 68 partners in the coalition and weaken it, according to European officials who spoke on the condition of anonymity because the deliberations are confidential.
He also made his presence felt at his first NATO summit, literally, pushing his way past Montenegro's prime minister, Dusko Markovic, whose country joins the organisation next month, in footage that went viral.
White House Press Secretary Sean Spicer brushed aside criticisms, calling them "almost laughable".
In almost 70 years of existence, Article 5 has only been invoked once - September 11, 2001 - and was the catalyst for NATO's involvement in the war in Afghanistan that followed.
President Donald Trump opened meetings with European Union leaders Thursday against the backdrop of striking anger from Britain over intelligence leaks and a decision by Manchester police to withhold information from the United States about the investigation into this week's bombing.
While some issues such as counter-terrorism were agreed upon, others remain uncertain.
Trump traveled Thursday morning to the European Union headquarters in Brussels for meetings with Donald Tusk, the president of the European Council, Jean-Claude Juncker, the president of the European Commission, and other EU officials. Trump has also been accused of sharing sensitive information supplied by Israel with top Russian diplomats. "This call for driving out terrorism is a message I took to a historic gathering of Arab and Muslim leaders across the region, hosted by Saudi Arabia", the president said.
However, he is likely to show support for the organization - especially after NATO Secretary General Jens Stoltenberg said he would support is calls for all member states to step up to the mark on defense spending and cooperation.
Tusk, a Communist-era dissident who once listed Trump among risks to the world order alongside Russia, China and Islamist violence, said he had tried to impress on the billionaire US president a need for Transatlantic cooperation to promote "values" like human rights and not just selfish "interests".