European Council President Donald Tusk | Ludovic Marin/AFP via Getty Images Donald Tusk: G7 summit may be last chance for unity European Council president warns of ‘difficult test’ for free world.

BIARRITZ, France — Leaders of the world's richest and most powerful democracies are increasingly unable to find "common language" and are at risk of getting caught up in "senseless disputes among each other," European Council President Donald Tusk warned Saturday before the start of a G7 summit.

"This is another G7 summit which will be a difficult test of unity and solidarity of the free world and its leaders," Tusk said at a news conference in the French city of Biarritz, where leaders will spend two days confronting the risk of a weakening global economy dragged down by trade wars, the threat of climate change, disputes with Iran and Russia, and an array of other vexing issues.

The G7, a club of the world's most advanced, industrialized democracies, has been under severely disrupted in recent years by U.S. President Donald Trump, who has sowed dissent — and injected unease and uncertainty — from the moment he arrived at his first leaders' summit in Taormina, Italy in 2017.

"There is still no certainty whether the group will be able to find common solutions, and the global challenges are today really difficult, or whether we'll focus on senseless disputes amongst each other," Tusk said at the news conference in the basement of a casino that is being used as one of the summit venues.

"The last years have shown that it is increasingly difficult for us to find common language when the world needs our cooperation more, not less," Tusk said.

"The burning Amazon rainforest has become another depressing sign of our times" — Donald Tusk, European Council president

"This may be the last moment to restore our political community," he warned darkly.

In his prepared remarks, and a brief question and answer session, Tusk said the EU would stick by the blockbuster Mercosur free-trade agreement that it has reached with a bloc of South American countries — despite French President Emmanuel Macron's threat to kill off the deal in response to fires that are raging in the Amazon rainforest.

Macron called the fires an "ecocide" and accused Brazilian President Jair Bolsonaro for misleading EU leaders at a G20 summit in Osaka, Japan in June — when they shook hands on the trade accord and he pledged to protect the rainforest.

Still, Tusk warned Bolsonaro that action would be needed to win ratification of the trade agreement.

"The burning Amazon rainforest has become another depressing sign of our times," Tusk said. "We of course stand by the EU-Mercosur agreement, which is also about protecting the climate and environment, but it is hard to imagine a harmonious process of ratification by the European countries as long as the Brazilian government allows for the destruction of the green lungs of Planet Earth."

Tusk added that the EU stood ready to help pay to fight the fires.

In some of his most pointed language, Tusk also called for an end to the trade wars largely instigated by Trump and noted angrily that persisting in such fights would only lead to a global economic downturn. Instead, he urged changes to the World Trade Organization as a better way to deal with disputes. In particular, he seemed to caution Trump against any escalation of a trade conflict with the EU after the U.S. president, en route to Biarritz, reiterated his threats of new tariffs against French wine and other EU goods.

"Trade deals and the reform of WTO are better than trade wars," Tusk said. "Trade wars will lead to recession, while trade deals will boost the economy, not to mention the fact that trade wars among G7 members will lead to eroding the already weakened trust among us."

On Russia's potential re-entry into the exclusive club of world powers, Tusk said Moscow, by continuing military aggression in eastern Ukraine, had done nothing to earn re-admittance. He suggested that G7 leaders instead invite Ukraine's president as a guest to next year's summit instead.

Tusk, who will meet the new British prime minister, Boris Johnson, on Sunday, dryly noted that it would be the "third British conservative prime minister" with whom he would discuss Brexit.

"The one thing I will not cooperate on is no deal" — Donald Tusk

Tusk is an avowed opponent of Brexit and there was no mistaking his jab over the political turmoil that the U.K. has suffered, leading to the departures first of David Cameron and then Theresa May.

"We will also be ready now to hold serious talks with Prime Minister Johnson," Tusk said, but he reiterated that EU position that it is incumbent on the British side to come forward with new ideas to avoid a no-deal crash out on the October 31 Brexit deadline, including any new proposal to replace the so-called backstop provision on the Irish border.

"The one thing I will not cooperate on is no deal," Tusk said. "And I still hope that Prime Minister Johnson will not like to go down in history as Mr. No Deal. We are willing to listen to ideas that are operational, realistic and acceptable to all member states, including Ireland, if and when the U.K. government is ready to put them on the table."