Trump Zings NATO, and EU Chief Zings Back

The meeting comes ahead of Mr Trump's four-day working visit to the United Kingdom, during which he will hold talks with Mrs May at Chequers, meet the Queen in Windsor Castle and spend two days in Scotland preparing for Monday's face-to-face summit with Mr Putin in Finland.

"There is a better-than 50-50 chance that the president will disrupt the North Atlantic Treaty Organisation summit, probably by complaining again that others aren't carrying their fair share of the burden, and possibly by threatening to withdraw if they don't step up on his terms", the official said.

Ahead of his first formal visit to Britain as US President, Mr Trump delivered a damning indictment of Mrs May's government saying it was in "turmoil" and Russian President Vladimir Putin may be less of a problem.

German Chancellor Angela Merkel and British Prime Minister Theresa May will join Trump for the two-day meeting, which comes during divisions among America and its allies over policies on trade and pulling out of the Iran nuclear deal.

Tusk reminded Trump that the first time NATO's mutual defense clause to assist other members under attack came after the terror attacks on 9/11 and that 870 European troops have fought and died in Afghanistan. Separately, the USA envoy said she's concerned that Russian Federation is trying to "flip" Turkey and other American allies to its column. "Frankly, Putin may be the easiest of them all".

"I'm going to tell NATO: You've got to start paying your bills".

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While the majority of the members are spending more, only five of them are meeting the agreed upon threshold of spending two percent of their GDP on defense.

Trudeau, who was called "weak" by Trump after the G7 meeting, pledged to boost Canadian troop numbers 540, from around 450 now.

Asked if Putin is a friend or foe, Trump said: "I really can't say right now".

While it's true that the U.S. provides roughly 22% of NATO's budget, this percentage is based off a carefully crafted formula that includes factors such as the size of a member state's economy. ("It's not us", Putin has told Trump, according to an American source, summarizing his message, "it's the subordinates fighting against our friendship.") Trump has also reportedly expressed to Putin his desire for better relations, and called the advisers who tried to prevent him from congratulating the strongman on his reelection "stupid people".

European officials have expressed hopes that member states can paper over their differences, but European Union President Donald Tusk delivered a blunt message to the U.S. leader on Tuesday. The U.S.is spending many times more than any other country in order to protect them. The EU hit back with tariffs on products including motorcycles and orange juice. Trump then heads to Helsinki to meet Putin.

Germany, Europe's largest economy, now spends 1.24% of GDP on defence.