Von der Leyen said she would present a "comprehensive plan" at an EU summit on Thursday, adding that "we have to have a surge in defence" and "step up massively".

"It's now of utmost importance we increase spending ... it's important we prepare for the worst," she added.

The leaders had a "good and frank discussion" and talked about the need for security guarantees, said the EU chief.

Addressing US President Donald Trump, she said: "We're ready together with you to defend democracy, to defend the principle that there's a rule of law, that you cannot invade your neighbour and bully your neighbour, or you cannot change borders with force".

"It's in our common interest that we prevent future wars," she added.