In tweeting complaints about Germany on Tuesday in apparent response to Merkel's questions about USA reliability, Trump casts the US not as the world's policeman but as its most temperamental bully.
The tit-for-tat row has escalated rapidly after Trump criticized major North Atlantic Treaty Organisation allies over their military spending and refused to endorse a global climate change accord at back-to-back summits last week.
Speaking at a joint news conference with Modi, Merkel reiterated her view that "we need to take our fate in Europe into our own hands".
But Merkel told supporters during a campaign event on Sunday that "the times in which [Germany] could fully rely on others are partly over".
Those comments, which caused shock in Washington, vented Europe's frustration with Trump on climate policy in particular.
German Chancellor Angela Merkel (L) sits next to Tunisia's President Beji Caid Essebsi (C) and speaks to President Donald Trump (R) as they attend a G7 expanded session during the G7 Summit in Taormina, Sicily, Italy.
Senior German politicians responded swiftly to his tweet.
The White House Press Secretary Sean Spicer, while addressing the reporters, said that Trump and Merkel get along "very well" and that their "bond" is growing which they had established during their talks at G7 Summit.
Spicer also disputed the media's interpretation of Merkel's remarks that she felt that Europe could no longer depend on the United States.
Haley on Germany: 'We have the backs of our allies'
Likewise, French President Emmanuel Macron's description of his odd, intense handshake with Trump in Brussels as a successful demonstration he wouldn't be intimidated by a US leader he likened to Russian dictator Vladimir Putin and the increasingly dictatorial Turkish President Recep Tayyip Erdogan may be quickly forgotten.
He also said that Ms Merkel's comment was not necessarily negative.
Merkel stressed that diplomacy is not a zero-sum game and that while Berlin was building strong ties with the rising Asian powers, transatlantic ties remained "of paramount importance".
But other leaders, such as Romania's President Klaus Iohannis, rebuked Merkel and insisted that the relationship with the USA remains vital.
Her spokesman, Steffen Seibert, said on Monday the chancellor was a "deeply convinced trans-Atlanticist" who believes in the idea of Western unity.
But Nikki Haley, the USA ambassador to the United Nations, told a television interviewer on Tuesday that "Germany will see in time that we have the backs of our allies".
"If it was a change in policy, he would have said it", Haley said.
"I say to all those for whom the future of our planet is important: let us continue on this path together so that we can succeed for Mother Earth", she said.More news: California bill would punish firms that bid on border wall
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