Like Canada, the European Union is preparing counter-measures against USA tariffs on steel and aluminium imports, in line with World Trade Organisation rules, Merkel said.
Her comments came amid an escalating fallout from a contentious meeting between Trump and other world leaders at the Group of Seven (G-7) Summit in Canada over the weekend. Trump then declared on Twitter that he no longer endorsed the statement.
On his way to Singapore, the USA president tweeted: "Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!"
"The withdrawal, so to speak, via tweet is of course. sobering and a bit depressing", Merkel said in an ARD television interview following the G7 summit.
Merkel open to European Union migration reform, cautious on eurozone
"We won't let ourselves be ripped off again and again", Merkel said.
However, Merkel avoided giving a definitive answer to the question of whether Germany will stop seeing the USA as an "important partner".
German Chancellor Angela Merkel looks towards US President Donald Trump, who arrived late for the Gender Equality Advisory Council working breakfast on the second day of the G7 Summit on June 9, 2018 in Quebec City, Canada.More news: Why me and my SNP MPs walked out of parliament today
The president's Twitter outburst, sent as he flew to an historic meeting with North Korea's ruler Kim Jong Un in Singapore, unravelled days of work by the world's top nations in attempting to resolve a global trade dispute.
Trade was a massive issue at this year's summit with Canadian Prime Minister Justin Trudeau and President Trump butting heads.
On Saturday Trump called Trudeau "very dishonest and weak" in a tweet after Trudeau repeated criticisms of U.S. tariffs on imported steel and aluminium and Trump's trade policies in a news conference following the Group of Seven meeting in Quebec.
Trump advisers Larry Kudlow and Peter Navarro nonetheless toed the presidential line, savaging Trudeau in Sunday show appearances over what Kudlow acknowledged were "things that the prime minister has said before, basically".More news: Luka Doncic Rumors: Real Madrid Star Will Remain in 2018 NBA Draft
The French president's office said in a statement: "International co-operation can not be dictated by fits of anger and throwaway remarks".
Canada's Justin Trudeau and other leaders had given closing press conferences and announced a US-backed joint statement.
Echoing Merkel's sentiments, French President Emmanuel Macron's office said that "international cooperation can not depend on fits of anger and a few words", according to a statement published by French public broadcaster RFI.More news: Trump Suggests Singapore Summit Ended North Korea Nuclear Threat