The vast undersea pipeline is opposed by the US and some other European Union members, who warn it could give Moscow greater leverage over Western Europe. Here is some of what he's said and how those statements stand up to the facts.
"The United States is spending far too much and other countries are not paying enough, especially some".
Of Nato's 29 members, just five meet that target this year: the US, Greece, Estonia, the United Kingdom and Latvia. The United States spends heavily to defend Germany from Russian Federation, he said, while Germany is paying "billions and billions of dollars a year to Russian Federation". Rather, they have committed to raise their own defense budgets to 2 percent of gross domestic product by 2024. That is more than current US spending as a proportion of GDP - and Pentagon budgets maintain bases and deploy fleets around the world, something that shouldn't be expected from Estonia or Spain.
On Tuesday he tweeted: "NATO countries must pay MORE, the United States must pay LESS".
"We're paying for anywhere from 70 to 90 percent to protect Europe, and that's fine.""Well they could increase it immediately, tomorrow, and have no problem", Trump said.
Meanwhile, NATO members in Europe contributed a total of $249.7 billion to their defense budgets and spent an average of 1.46% of GDP.
Trump has spent weeks berating members of the alliance for failing to increase military spending, accusing Europe of freeloading off the USA and even raising doubts about whether he would come to members' defense as required if they were ever attacked.
Still, it's significantly more than the next closest country.
If Mr. Trump's real concern were inadequate European spending, he should have started by better informing himself. "I think that two world wars and the Cold War taught us that we are stronger together than apart", he told the president, trying to calm tensions. "Now maybe there's a bit more urgency now because he's blunter than his predecessors in criticizing his European partners".
NATO Secretary-General Stoltenberg later told reporters that Trump had used "very direct language" but that all NATO allies were agreed that the cost of defence spending must be spread around and that a year ago had seen the biggest increase in a generation.
Stoltenberg responded by emphasizing NATO's unity.