EU President Donald Tusk said Tuesday that ex-London mayor Boris Johnson suffers from "political amnesia" for recent comments in which Johnson said the European Union's goals are similar to those of Adolf Hitler.

Tusk - speaking after talks in the Danish capital of Copenhagen with Danish Prime Minister Lars Lokke Rasmussen - said the wide range of challenges facing EU member states are best solved jointly, a strong rebuttal to Johnson, who backs Britain's departure from the EU.

"When I hear the EU being compared to the plans and projects of Adolf Hitler, I cannot remain silent," Tusk said.

"Such absurd arguments should be completely ignored if they hadn't been formulated by one of the most influential politicians of the ruling [Tory] party," Tusk added, referring to Johnson.

Johnson, a conservative politician, favours Britain leaving the 28-nation bloc in a referendum next month. In remarks at the weekend to The Sunday Telegraph newspaper, Johnson compared the EU to failed attempts to unify the continent, such as the one attempted by Hitler when he ruled Nazi Germany.

The former London mayor "crossed the boundaries of a rational discourse, demonstrating political amnesia," Tusk said.

At a news conference with Rasmussen, Tusk said that "today's atmosphere of uncertainty is fuelled by crises and tensions which have not been provoked by the European Union: the massive influx of migrants, global economic turmoil, terrorism or the aggressive Russian policy."

He added he was "convinced that the European Union is our common tool to solve problems," noting that the individual countries face these challenges "also without the EU."

While the EU may be blamed for many problems, "it still remains the most effective firewall against the ever dangerous and often tragic conflicts among the nations of Europe," Tusk said.

"The only alternative to the union is political chaos, the return to national egoism and, in consequence, the triumph of anti-democratic tendencies, which can lead to history repeating itself," Tusk said.

The two leaders were later to fly to Greenland to study the effects of global warming.

"Nowhere else in the world is climate change more visible than in the Arctic," Rasmussen said, citing receding glaciers and sea ice.

Greenland has a high degree of autonomy within Denmark. The two leaders were to meet with Greenland premier Kim Kielsen.

Rasmussen said he had raised the topic of Denmark's interest in seeking a parallel agreement with the EU to remain in the cross-border crime fighting agency Europol.

In December, Danish voters voted to keep the country's long-standing opt-out from EU justice and home affairs rules.

Denmark's exemption from EU matters related to justice and home affairs was introduced 1993. But, since the 2009 Treaty of Lisbon, more EU cooperation has been transferred to a supranational level in reforms also affecting Europol, forcing the need to realign Denmark's membership.

While the EU wanted to keep Denmark "as close to its core as possible, at the same time we must respect the outcome of national referendums as well as existing political and legal limitations," Tusk said.

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