Governments in Europe are under pressure to step up on defence as US President Donald Trump questions the central role the United States has held in NATO since the wake of World War II while making overtures towards Russia on Ukraine.

"By 2030, Europe must have a strong European defence posture," von der Leyen said in a speech to cadets at the Royal Danish Military Academy in Copenhagen.

"If Europe wants to avoid war, Europe must get ready for war," the European Commission chief warned -- setting a target for the continent to "have re-armed and developed the capabilities to have credible deterrence".

Von der Leyen's speech came a day before the commission presents a white paper to flesh out proposals aimed at helping Europe rearm, as part of a broader roadmap that she dubbed "Readiness 2030".

Bolstering Europe's defences is to top the agenda at a leaders' summit later this week, after the EU endorsed a commission plan aimed at mobilising up to 800 billion euros ($875 billion) in investments.

The plan -- which the white paper builds on -- proposes easing the bloc's fiscal rules to allow states to spend much more on defence, a measure the commission says could potentially unlock 650 billion euros over four years.

It proposes providing states with EU-backed loans of up to 150 billion euros.

And it allows member states to repurpose so-called "cohesion" funds intended for the development of poorer European countries and lifts curbs on defence investments by the bloc's lending arm, the European Investment Bank.

Further steps, von der Leyen indicated in Copenhagen, would involve helping states to pool European demand and ramp up joint procurement, with a view to strengthening Europe's defence industrial base.

She promised to convene a "strategic dialogue" with the sector to identify the obstacles they face.

And the commission chief reiterated her call to ramp up support for Ukraine -- in what she has branded a "steel porcupine strategy".

"We need to make Ukraine strong enough to be indigestible for potential invaders," she said, while also emphasising that Europe was "learning from Ukraine's defence industry" with its battlefield experience and capacity for swift innovation.