"We must remember that every doubt we show here in Europe is a trench that Russia will definitely try to occupy," he told a European summit in Moldova.

Ukraine is a candidate to join both NATO and the European Union, and Moldova the EU, but some European capitals are wary of setting a formal timeline for membership as Russia's invasion continues.

But, addressing four dozen European leaders from inside and outside the union and the alliance, the Ukrainian leader warned that delaying a decision would undermine the West's strength.

He complained that if Ukraine and Moldova have no clear path to joining the groupings, vulnerable pro-Western political forces in Belarus and Georgia will be at risk.

"Whether these doubts are about vital security steps or doubts about our unity, maybe about our ability to meet the challenges of our time, every doubt brings more insecurity," he said.

NATO foreign ministers were separately in Oslo debating what level of promises or security guarantees could be granted to Ukraine at July's alliance summit.

"I would like to emphasise in summer in Vilnius at the NATO summit, clear invitations to NATO membership are needed, and security guarantees all the way to NATO membership are needed," Zelensky said, adding that he wanted a clear invitation to join the EU.

"Recently, a Chinese journalist asked me 'Why NATO?' and the answer is very simple. When there are no security guarantees, there are only war guarantees," he said.

"We need a just peace. That is why every European country that borders Russia, and that does not want Russia to tear them apart, should be a full member of the EU and NATO.

"And there are only two alternatives to this: Either an open war or creeping Russian occupation."