Scandinavian leaders, traditionally reserved on nuclear matters, gave the proposal a cautious welcome but Denmark's Prime Minister Mette Frederiksen said that in the current security circumstances, "everything must be on the table now".

Leaders of the 27 European Union states are in Brusseles for talks with Ukrainian President Volodymyr Zelensky and aim to cement support for the war-torn country after US President Donald Trump's freeze on military aid to Kyiv.

President Emmanuel Macron announced on Wednesday that he would discuss extending France's nuclear deterrent to other European nations.

"This readiness of France, this is something very promising," Tusk told reporters in Brussels, adding: "We have to treat this proposal seriously."

He also reiterated a call for Europe to strengthen its defences against Russia.

"It must be one of our priorities to coordinate all our capacities in Europe and to build, in fact, one well-coordinated military power. This is what can give us a clear advantage towards Russia," Tusk said.

"Europe must take up this challenge, this arms race, and it must win it," he added.

Poland, a NATO member and a key ally of its neighbour Ukraine, also shares a border with Russian exclave of Kaliningrad, and Belarus, Moscow's ally.

Macron said he would open a debate on extending France's nuclear deterrent, following a call from Germany's likely next chancellor Friedrich Merz.

Merz, whose right-wing party won Germany's elections, said last month he wanted a discussion on "nuclear sharing" with France and Britain, Europe's only nuclear powers other than Russia.

In April, Poland's President Andrzej Duda said his country was ready to host NATO's nuclear arms. Moscow in response warned it would take steps to "ensure its security".

Denmark's Frederiksen said her country was open to discussions on Macron's proposal. "It's not something that we're working on but you won't hear me say no to other people's ideas."

Sweden's Prime Minister Ulf Kristersson also welcomed the debate.

"Like most people, the Swedes want to have as few nuclear weapons as possible but at the moment we should be happy and grateful that two of our neighbours (France and the UK) have nuclear weapons," he said.