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Using his favourite form of communication, Mr Trump tweeted today: "We have a MASSIVE trade deficit with Germany, plus they pay FAR LESS than they should on NATO & military. "Very bad for U.S. This will change." The billionaire leader did not clarify how that would change.

Getty•Twitter Donald Trump tweeted out his indignation for Germany after meeting Angela Merkel

On Monday Chancellor Merkel underlined her doubts about the reliability of the United States as an ally. She vented European frustrations with Donald Trump after back-to-back summits last week, but said she was a "convinced trans-Atlanticist".

Getty Donald Trump and Angela Merkel were not on the best of terms at the G7 summit in Sicily

Speaking in a packed beer tent in Munich on Sunday, after a Group of Seven summit in Sicily and a NATO meeting in Brussels - both dominated by tensions with Mr Trump - Mrs Merkel spoke with surprising frankness. She said: "The times when we could fully count on others are over to a certain extent. I have experienced this in the last few days. "We Europeans must really take our destiny into our own hands, of course in friendship with the United States, in friendship with Great Britain, with other neighbours wherever possible, also with Russia," she continued. "But we must know that we need to fight for our future ourselves, as Europeans, for our destiny."

World leaders gather at G7 summit 2017 Fri, May 26, 2017 Donald Trump will face Theresa May and world leaders at the G7 summit in Sicily Play slideshow EPA 1 of 8 G7 Summit in Taormina

Her comments over the weekend sent shockwaves through Washington. Mr Trump backed a pledge to fight protectionism at the end of a summit of the G7 group of wealthy nations on Saturday. However, he refused to endorse the climate pact, saying he needed more time to decide. Martin Schulz, former President of the European Parliament and chairman of Mrs Merkel's coalition partner party the SPD, backed the German Chancellor, accusing Mr Trump of being an "autocratic leader" who was trying to "humiliate" her.

Mrs Merkel added ties with the United States were of "paramount importance", but she otherwise stuck to the thrust of her Sunday message from a packed Munich beer tent. At a conference on sustainable development, she said: “It became clear at the G7, when there was no agreement with the USA, how long and rocky this path would be. "I think it was good not to gloss over the differences." Mrs Merkel indirectly warned Mr Trump he risked isolating the United States. She said: "Anyone who today puts on national blinkers and no longer has eyes for the world around him is, I am convinced, ultimately out on a limb."

Getty Mrs Merkel hit out at Mr Trump on Sunday during a beer festival

Getty The G7 meeting was "very unsatisfactory", Mrs Merkel said

Her spokesman, Steffen Seibert, told reporters Mrs Merkel felt it was right to flag differences in Germany's ties with the United States in order to maintain healthy relations. He said: ”Because trans-Atlantic relations are so important to this chancellor, it is right from her viewpoint to speak out honestly about differences.” Mr Seibert, added, the trans-Atlantic ties "are a firm pillar of our foreign and security policy" Interior Minister Thomas de Maiziere stressed Germany's "excellent" security ties with the United States.