Trump is expected to slam the alliance as he calls for European nations to spend more on defence.
The president appeared to be referring to the Nord Stream 2 pipeline that would bring gas from Russian Federation to Germany's northeastern Baltic coast, bypassing Eastern European nations like Poland and Ukraine and doubling the amount of gas Russian Federation can send directly to Germany.
Trump predicted as he departed Washington that the "easiest" leg of the journey would be the sit-down with Putin - a comment that did little to reassure allies fretting over his potential embrace of a Russian leader they regard as troublesome. Budget increases started after Russia's 2014 annexation of Ukraine's Crimean Peninsula, and they have accelerated in the Trump era in response to the US President's criticism.
He vowed not to be "taken advantage" of by the European Union, which he accuses of freeloading by relying on the United States for its defence while blocking USA imports into the bloc, the world's biggest market.
"I think we can cope with it", von der Leyen said. -NATO meeting ahead of the summit, according to Reuters.
The European Union is firing back at President Donald Trump's criticisms. On 17 May, the Wall Street Journal reported that Trump is demanding that Germany drop Nord Stream 2 as one of the conditions for a trade deal with Europe that would not include high tariffs on steel and aluminium.
President Trump will be in Europe this week to meet with world leaders - allies and adversaries alike - to discuss pivotal agenda items for American, NATO, and global stability interests.
Though she said that Germany did have "a lot of issues with Russian Federation", she noted that it was important to "keep the communication line between countries or alliances and opponents" open.
And he vowed not to be "taken advantage" of by the European Union, which he accuses of relying on the United States for defence while restricting U.S. imports into the bloc, the world's biggest market.
"It's certainly going to be an interesting time with NATO", Trump told journalists. Very supportive. And maybe we'll speak to him when I get over there.
Mr Trump said he had not spoken with Mrs May, adding: "Boris Johnson is a friend of mine".
Trump's weeklong trip to Europe will continue with a stop in Scotland before ending with a sit-down in Helsinki with Putin.
"I think these countries have to step it up, not over a 10-year period, but they have to step it up immediately", Trump said, pointing to Germany in particular as a "rich country" that "could increase (defense spending) immediately tomorrow and have no problem".
"We're protecting Germany, we're protecting France, we're protecting all of these countries".
"Frankly Putin might be easiest of them all".
Mr Tusk also made the point that the United States did not and would not have a better ally than the EU, reminding the president that it was European troops who had fought and died in Afghanistan after the 11 September 2001 attacks on the US.
'You tell me if that's appropriate because I think it's not.
The NATO summit is also being attended by leaders of so-called NATO partner states that are not members of the alliance.