Merkel said that discussions at the July 7-8 gathering of world leaders in Hamburg would be difficult given Trump's climate scepticism and "America First" stance, but that she was determined to seek a clear commitment for the Paris accord against global warming and a pledge against protectionism.
When Trump announced in early June he would withdraw from the Paris deal, "we knew that we could not expect discussions to be easy" at the G20 summit, Merkel told the German parliament.
"The differences are obvious and it would be dishonest to try to cover that up. That I won't do," she said, adding that the US exit from the 2015 Paris pact had made Europe "more determined than ever" to make the accord a success.
Without naming names, she also warned that "those who think that the problems of this world can be solved with isolationism or protectionism are terribly wrong" and pledged to seek a "clear signal for open markets and against sealing off" at the summit.
Trump's divergent stance has left Western allies struggling to find a common front for the G20 gathering -- unlike previous summits, when differences were drawn along global north-south and east-west lines.
With Russian President Vladimir Putin, Turkey's Recep Tayyip Erdogan and Saudi Arabia's King Salman in the volatile mix for the power meet, the list of potential minefield issues also includes the Syrian war, the Ukraine conflict and the diplomatic shut-out of Qatar.
Trump's anti-immigration stance has also emboldened many of the EU's ex-communist members in the east, which have staunchly opposed Merkel's pleas to accept larger shares of the refugees who have flocked to Europe.
With the fault lines multiplying, Merkel was to meet with her western European allies later Thursday to draw up a common battle strategy.
European cohesion
"Merkel has called a summit between Europeans because there is a problem with the relationship with Trump," said a diplomatic source, speaking on condition of anonymity.
"It's necessary to ensure European cohesion because within the G20, it's complicated."
Besides the transatlantic differences, "there is also a new European division growing between east and west," noted Jean-Dominique Giuliani, president of the Fondation Robert Schuman, a Paris-based think-tank.
The problems were buried when wealthier EU members supported the east with "financial flows towards central and eastern Europe", he said, adding that "they are reappearing again on the question of refugees".
Threatening to deepen divisions, Trump will head to Warsaw for a summit of central and eastern European leaders, likely to include Hungary's hardline Prime Minister Viktor Orban, a day ahead of the G20.
European affairs journal Eurativ warned that "as some EU countries shun him and others welcome him with open arms, Trump could become the wedge that drives the Union apart".
'France is back'
Even Merkel's European guests -- the leaders of G20 members France, Britain and Italy, as well as of invited countries the Netherlands, Spain and Norway -- have very different relationships with Trump.
Britain's Theresa May, who is leading her country out of the EU, has been derided at home for seeking to curry favour with the US leader, after she invited him for a state visit that sparked a national outcry.
French President Emmanuel Macron, who had been dubbed an anti-Trump by some with his strong pushback against Trump's climate stance, this week invited the US president to attend July 14 Bastille Day celebrations.
"With Macron, France is back, there is a re-balancing that was necessary with the relationship with Germany," Giuliani said.
Ahead of the meeting, German Foreign Minister Sigmar Gabriel said it was important for Europe to face up to the US confidently.
"The German government does not have an anti-US strategy, but in America, there are strategists who are planning an anti-Europe, anti-German agenda," he said.
"We do not want to forcefully separate the US from Europe. But what we don't want either is to appear like an appendage of US policies."
