The EU's 27 leaders convened the Brussels talks involving Britain's prime minister and the head of NATO to brainstorm ways to ramp up European defence spending -- a key demand that President Donald Trump has made to America's allies.

But it was Trump's repeated threat to target Europe "soon" -- after having ordered tariffs on Canada, Mexico and China -- that set the meeting's tone.

"If we are attacked in terms of trade, Europe -- as a true power -- will have to stand up for itself and therefore react," French President Emmanuel Macron warned.

The tough talk mirrored the message from the European Commission, which leads trade policy for the bloc and warned Sunday it would "respond firmly" to any US tariffs. German leader Olaf Scholz likewise said Europe should be ready to act.

The EU's foreign policy chief, however, struck a more conciliatory note.

"We need America, and America needs us as well," Kaja Kallas told reporters. "There are no winners in trade wars."

Poland's Prime Minister Donald Tusk, whose country currently holds the EU presidency, put it more bluntly, labelling trade wars "totally unnecessary and stupid".

Trade aside, Trump has rattled US allies with a series of direct threats -- not least his insistence that he wants to acquire strategically important Greenland, an autonomous Danish territory.

Denmark's prime minister, Mette Frederiksen, reiterated in Brussels that the Arctic island was "not for sale".

- 'Never always tranquil' -

The trade threats from the White House add an unwelcome new layer to the already complex challenge of bolstering European defences -- faced with a menacing Russia and the spectre of Washington pulling back.

Trump has made clear Europe can no longer take US protection for granted, insisting that NATO countries more than double their defence spending target to five percent of their total economic output -- a goal out of reach for many.

He has also vowed to bring a quick end to Russia's war in Ukraine, leaving Europeans fearful he could sideline them and force Kyiv into a bad deal.

NATO chief Mark Rutte insisted the trade tensions would not weaken the alliance's collective deterrence.

"There are always issues between allies -- it is never always tranquil and happy going," he said.

European nations have ramped up their military budgets since Russian President Vladimir Putin launched the all-out invasion almost three years ago.

But EU officials concede they are still not arming themselves fast enough as warnings grow that Moscow could attack one of their own in the coming years.

- 'Crucial' -

There is widespread consensus across Europe on the need to step up on defence, with Brussels estimating the needs at 500 billion euros ($510 billion) over a decade.

"It's crucial that we today start real discussions how we can and how much we strengthen," Finland's Prime Minister Petteri Orpo said.

Key dividing lines revolve around how to fund investment, whether EU cash should be spent only on EU arms, and NATO's role.

Some members are pushing for massive joint EU borrowing but Germany -- facing a fraught election -- has tried to shut down the sensitive discussion.

With no sign of movement on that, EU states have called for the bloc's lending arm, the EIB, to drop limits on lending to defence firms.

On weapons, France -- long accused of caring more for its own industry -- insists arms should be bought in the EU.

- UK security deal? -

As doubts swirl over the transatlantic relationship, many are keen to step up ties with an old friend: Britain.

Prime Minister Keir Starmer will be back in the fold -- at least for one dinner -- as the first UK leader to attend a European Council gathering since the country withdrew from the EU five years ago.

Starmer said he wanted to work with EU leaders to "crush Putin's war machine" by further targeting Russia's economy.

The British leader, who has sought to reset relations after the rancour of Brexit, said he wanted to strike a "ambitious" security partnership with the EU.

That could bring Britain, with its potent military and large defence industry, a little closer.

"We can't be commentators when it comes to matters of peace on our continent," Starmer said.

But the bitter legacy of Brexit remains.

Numerous EU diplomats said there cannot be progress until a dispute over fishing rights is resolved and London drops its opposition to a youth mobility scheme proposed by Brussels.