US President Donald Trump, who has paused crucial US military aid to Ukraine, has ruled out the notion of Kyiv joining NATO, as has Moscow.

The US president is keen to end the war but has refused to give Ukraine the security guarantees it has demanded to deter Russia from attacking again.

In an interview with US news channel CNN late Monday, Stubb proposed that Ukraine automatically become a NATO member if Moscow were to violate any ceasefire agreement agreed by the warring sides.

"Why don't we instead give them a de facto NATO membership, which basically means that when the negotiations end and we have a peace settlement, a part of it all is to say that if Russia ever breaks that settlement, breaks the ceasefire or attacks Ukraine again, Ukraine becomes automatically a member of NATO," he said.

"I think that is a strong enough backstop," he added, referring to Europe and Ukraine's insistence that the US ensures a peace accord is respected.

In mid-February Republican US Senator Lindsey Graham, a Trump ally, first floated the idea on the sidelines of the Munich Security Conference.

"If you can't get them into NATO right now, my idea ... is to be really clear: Tell (Russian President Vladimir) Putin if you ever do this again, if there's another incursion by Russia into Ukraine, that leads to automatic admission into NATO by Ukraine as a tripwire," Graham told US news outlet Politico.

Trump's dramatic end to Putin's isolation has forced European leaders to boost their defence cooperation and find their own path forward to end the war in Ukraine.

An EU summit in Brussels on Thursday was to focus on Ukraine and European security issues.

EU Commission President Ursula von der Leyen on Tuesday presented a five-part plan to mobilise some 800 billion euros for Europe's defence, and help provide immediate military support for Ukraine after Washington suspended aid.