Trump G7 tweets 'sobering and depressing': Merkel

US President Donald Trump (right) talking with German Chancellor Angela Merkel (centre) and surrounded by other world leaders during a meeting of the G7 Summit in Canada. (AFP/Jesco DENZEL)

"It's hard, it's depressing this time, but that's not the end" of the Group of Seven, she said in a rare one-on-one interview with ARD public television.

"I don't want us to keep inflating our language," she added, saying the word "depressed" was "already a lot, coming from me", in an ironic reference to her usual unflappable appearance.

Trump departed early from the Quebec meeting on Saturday to head to Singapore for his summit with North Korea's leader Kim Jong Un.

Soon afterwards, Trump rejected in a series of angry tweets from Air Force One the text of a G7 consensus statement - traditionally a paean to shared Western values and objectives under American leadership.

Asked about Trump's threat to target US tariffs against cars - a vital industry for Germany which supports over 800,000 jobs - "we will have to think again about what we'll do," Merkel said.

"Hopefully the European Union will again act as collectively as it has this time," the chancellor said.

The EU has already said it will announce counter-measures against Trump's metals tariffs on Jul 1.

Meanwhile, Merkel brushed aside Trump's suggestion that Russia should be re-admitted to the group of the world's leading economies.

Russia has been shut out since its 2014 annexation of the Crimean peninsula from Ukraine, and has since earned Western condemnation for its military intervention in Syria in support of dictator Bashar al-Assad.

"We need Russia for disarmament talks" and for discussions about both Ukraine and Syria, Merkel said. "There is no question about that."

But "I understand the G7 to be a group for countries that have shared principles," she added.

Looking ahead to the next major diplomatic encounter with Trump - a meeting of NATO alliance leaders in July - "it won't be easy", Merkel said.

The US leader has blasted Europeans, Germany above all, for failing to spend enough on their own defence, claiming they are taking advantage of US military power.

To deal with future military threats to Europe, Merkel pointed out that the EU had already agreed to "build up complementary capabilities to NATO".

The bloc must develop "a joint strategic culture", she added, "otherwise Europe will be ground up in a world with very strong poles" of power elsewhere.