"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.
Trump will have his first standalone summit with Putin in Helsinki, Finland, Monday.
Stoltenberg has the challenging task of chairing the first major gathering of western leaders since a Group of Seven meeting last month ended with Trump insulting the host, Canadian Prime Minister Justin Trudeau.
Mrs May has been strongly criticised over her Chequers plan for Brexit which Jacob Rees Mogg has claimed will leave Britain as a "vassal state" to the European Union and Mr Johnson has said will turn the United Kingdom into an European Union "colony". According to an analysis last month, the USA share of NATO's total civil and military budget is 22 per cent, or about one-fifth of the total, compared with 10 per cent for the United Kingdom and France and less than 15 per cent for Germany and the rest from other alliance members. Separately, the US envoy said she's concerned that Russian Federation is trying to "flip" Turkey and other American allies to its column.
Trump's Twitter post on Tuesday echoed those on Monday and his criticism at a rally of supporters in the United States last week in which he said that Washington was unfairly carrying nearly all the cost of defending Europe. 'I'm confident the UK-US alliance will play a leading role in advancing our shared values of democracy and freedom, ' he said. "We're being taken advantage of by the European Union", Trump said, adding that US NATO spending "helps them a lot more than it helps us".
At the time, Johnson was Mayor of London and said Trump was talking "complete and utter nonsense".
"Europeans spend on defence many times more than Russia and as much as China, and I think you can have no doubt, Mr President, that this is an investment in common American and European defence and security, which cannot be said with confidence about Russian or Chinese spending".
He also tweeted, "NATO countries must pay MORE, the United States must pay LESS".
While Germany's spending is increasing, the country will not reach the target by 2024, instead expecting to boost spending to 1.5 percent by 2025. The EU hit back with tariffs on products including motorcycles and orange juice. "Who would think?" Trump said.
"The only reason I wouldn't go to some parts of NY is the real risk of meeting Donald Trump".
The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied the allegations of election meddling, in spite of evidence to the contrary. "There has been this tension, which has been a long-running factor ... but I'm pretty positive on the relationship overall", the State Department official said.
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