BRUSSELS -- Environmental campaigners protested Friday against U.S. President Donald Trump's decision to pull the United States out of the Paris climate accord, while nations around the world pledged to double down on their efforts to curb global warming in response to the U.S. move.

In Berlin, Greenpeace activists projected Trump's silhouette onto the side of the U.S. embassy along with the words ".TotalLoser, so sad!"

Hours later German Chancellor Angela Merkel summoned reporters for an impromptu statement in which she called Trump's decision "extremely regrettable, and that's putting it very mildly."

But Merkel, whose country hosts this year's international climate summit, said it was now time to look ahead.

"This decision can't and won't stop all those of us who feel obliged to protect the planet," she said. "On the contrary. We in Germany, Europe and the world will combine our forces more resolutely than ever to address and successfully tackle challenges for humanity such as climate change."

In Brussels, European Council President Donald Tusk, standing alongside Chinese Premier Li Keqiang, said that the EU and China "are convinced that yesterday's decision by the United States to leave the Paris agreement is a big mistake."

Referring to "the latest unfortunate decisions of the new administration," Tusk said that the EU and China had "demonstrated solidarity with future generations and responsibility for the whole planet" by reaffirming their climate change commitment.

Anticipating a possible U.S. pullout, officials from China and the European Union -- two of the world's major polluters -- have prepared a declaration reaffirming their commitment to the 2015 Paris Agreement, which is widely considered a landmark deal for bringing together almost all countries under a common goal.

Trump said the United States would be willing to rejoin the accord if it could obtain more favourable terms, but the leaders of France, Germany and Italy said in a joint statement Thursday that the agreement cannot be renegotiated, "since it is a vital instrument for our planet, societies and economics."

Germany's environment minister, Barbara Hendricks, told reporters in Berlin that other countries will fill the leadership vacuum but none will be expected to make up the shortfall in emissions reductions caused by Washington's exit.

Hendricks said the absence of $500 million contributions from the United States to the Green Climate Fund will be felt from 2018, but suggested the gap could be filled with "other financing mechanisms, for example through the World Bank."

The Green Fund is designed to help poor countries adapt to climate change and bypass some of the heavily polluting technologies formerly used by rich countries.

Poor countries are predicted to be among the hardest hit by global warming, with some foreseeing tens of millions of "climate refugees" in coming decades.

South Africa called the U.S. pullout "an abdication of global responsibility. "It said the U.S. has a "moral obligation" to support poorer countries in the global effort against climate change.

In Tokyo, Japan's environment minister, Koichi Yamamoto, said: "I'm not just disappointed, but also feel anger."

Former Mexican President Vicente Fox also criticized Trump's move, saying on Twitter: "He's declaring war on the planet itself."

A top atmospheric scientist at the U.N.'s weather agency said Friday that the "worst-case scenario" caused by the planned U.S. pullout from the Paris climate deal would be a further 0.3-degree Celsius (0.5 Fahrenheit) rise in global temperatures by 2100.

Deon Terblanche of the World Meteorological Organization said the organization hasn't run any new scientific models following Trump's announcement.

The Paris accord aims to prevent average temperature around the world from heating up by more than 2 degrees Celsius (3.6 degrees Fahrenheit) before the end of the century, compared to before the start of the industrial age.

Scientists say every fraction of a degree change in average temperatures can lead to noticeable swings in local weather patterns, though consequences are difficult to predict.

Jordans contributed to this report from Berlin. Jamey Keaten in Geneva, Cara Anna in Johannesburg and Raf Casert in Tallinn, Estonia, contributed.