U.S. President Donald Trump hit back at Germany's Chancellor Angela Merkel Tuesday as the war of words between the two leaders intensified following a G-7 leaders' summit in Italy this weekend.

Trump expressed his views on U.S.-German trade and the country's spending commitments to NATO in a Tweet sent early Monday that suggests relations between the two leaders are quickly deteriorating.

Germany's trade surplus with the United States stood at just under $65 billion last year, according to U.S. government data, and is hovering around $14 billion after the first three months of 2017, making it the fifth largest in terms of U.S. deficits.

Trump's criticism follows a stunning speech by Merkel in Munich Sunday during which the German leader said Europe could no longer reply on its traditional allies and that "we must fight for our future on our own, for our destiny as Europeans."

"I have experienced this in the last few days," Merkel told an audience of around 2,000 supporters in a local beer tent. "And that is why I can only say that we Europeans must really take our fate into our own hands - of course in friendship with the United States of America, in friendship with Great Britain and as good neighbours wherever that is possible also with other countries, even with Russia."

Her assessment was also supported by her chief political rival, former European Parliament President Martin Schulz, who told supporters that Trump had "humiliated" Merkel and other European leaders during the G-7 summit.

"I reject with outrage the way this man takes it upon himself to treat the head of our country's government," Schulz said. "That is unacceptable."

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