"NATO allies have agreed that Ukraine will become a member of our alliance, but at the same time that is a long-term perspective," Stoltenberg told reporters during a visit to Finland's capital Helsinki.

Stoltenberg added that "the issue now is that Ukraine prevails as a sovereign independent nation, and therefore we need to support Ukraine."

After Russia's invasion, Ukraine's President Volodymyr Zelensky has urged the US-led military alliance to grant his country a fast-track membership.

Ukraine applied for EU membership in February 2022, shortly after it was invaded, and was granted candidate status in June.

When the war ends "we need to ensure that history doesn't repeat itself," Stoltenberg told a press conference with Finnish prime minister Sanna Marin.

"President Putin cannot continue to attack neighbours. He wants to control Ukraine and he is not planning for peace, he is planning for more war."

Spooked by Moscow's invasion of Ukraine, Finland and Sweden dropped their decades-long policies of military non-alignment and applied to join NATO in May of 2022.

"I see that the future of Ukraine is to be part of the European Union and also a member of NATO," Marin added.

Turkey and Hungary are the only remaining members yet to ratify the Finnish and Swedish bids to join the alliance.

Stoltenberg said that "both Finland and Sweden have delivered on what they promised in the trilateral agreement they made with Turkey last June in Madrid."

"The time is now to ratify and to fully welcome Finland and Sweden as members," he said.