European leaders said on Thursday that they would work together to press their views on climate change and free trade at a G20 summit next week.
They however, made clear that they would not try to isolate US President Donald Trump who has split with them on both issues.
After a meeting in Berlin hosted by German Chancellor Angela Merkel, she said European countries wanted to send a message of "determination" at the summit in Hamburg on July 7-8 but also find "common solutions" with Washington on issues such as climate change.
French President Emmanuel Macron said he hoped the US would "return to reason" after announcing it would pull out of the Paris climate accord.
He however added that it did not make sense to isolate Trump, saying Europe shared "a lot" with Washington, including the view that terrorism must be tackled.
Earlier in the day, speaking at German parliament, Merkel appeared poised to head for a collision course with Trump as she vowed to have climate change as one of the central tasks of the upcoming G20 summit.
Merkel said the climate change scepticism of the Trump administration made it all the more important for the European Union to show leadership.
"Since the decision of the US to quit the Paris climate agreement, we are more determined than ever to make it successful," she said.
We must tackle this existential challenge, and we cannot wait until every last person on earth has been convinced of the scientific proof.
When chairing the summit, Merkel said she would seek to guide talks such that they furthered the goals of the Paris deal, but she conceded that differences with the US meant discussions would not be easy.
She said the forum, which meets on July 7 and 8, would also discuss common approaches to trade, another area in which the Trump administration's protectionist instincts are at odds with the European Union's.
"Anybody who believes the problems of the world can be solved with isolationism and protectionism is labouring under a huge error," Merkel said.
Far from being weakened by Britain's vote to quit the EU, the bloc would remain united, she added, promising a renewed drive with France's President Emmanuel Macron to deepen cooperation within the bloc and the Eurozone.