The U.S.is at 3.5 percent, or $685 billion past year.
The president made the remarks less a formal proposal, but while urging members to up their contributions and promote fair burden sharing across the organization.
Striking a strident tone at the summit, Trump's aspirational target of 4 percent of gross domestic product (GDP) was above the United States own spending on defense. American officials say the U.S.is itself short of that mark, spending 3.3 percent of its GDP on defense.
At issue, the disparity in defense spending; Kevin Corke reports from Brussels, Belgium.
"Military spending for any country", Koshagarian said, "should be determined by two things: real national security needs, and curbing waste and profit motives that drive military spending up without making us safer".
As the day began, Trump wasted no time going on the offensive as he began a week of high-stakes diplomacy on both sides of the former Cold War divide.
He said: "Why are their (sic) only 5 out of 29 countries that have met their commitment?" According to Stoltenberg, eight countries would meet the 2 percent target this year and a "majority" put forward plans to do so by 2020 - compared to only three countries who were meeting the commitment in 2014.
Trump asserted earlier Wednesday that Germany is "totally controlled" by and "captive to Russia" because of a pipeline project.
Merkel pushed back, insisting that Germany makes its own decisions.
Trump has preferred to take aim at allies.
"And, indeed, we had an opportunity to have an exchange about economic developments, on issues such as migration, and also the future of our trade relations", she said.
Donald Trump is once again attacking North Atlantic Treaty Organisation...while at the North Atlantic Treaty Organisation summit.
"It was also because of your leadership", Stoltenberg told Trump.
Mr Trump said: "Germany is a captive of Russian Federation".
"I think it's very sad when Germany makes a massive oil and gas deal with Russia", Trump said.
Trump's comments were "ill-informed", said Sen. "Explain that. And it can't be explained, you know that".
Trump's decision to meet Putin was widely regarded as a blow to the prime minister's attempts to isolate Russian Federation following the Salisbury attack in March, even though he had strongly condemned the Kremlin's actions at the time. Munich's Süddeutsche Zeitung headlined its story: "It is not only bad, it is catastrophic". "So we're supposed to protect Germany, but they're getting their energy from Russian Federation". Former German Chancellor Gerhard Schroeder is a top executive at the Russian-Government-controlled company that runs the Nord Stream 2 pipeline.
Trump's aides seated around the table, including chief of staff John Kelly and the USA ambassador to NATO, Kay Bailey Hutchison, looked visibly uncomfortable at points. But the idea is that President Trump is talking a lot, but not really doing a lot.