Nato chief Jens Stoltenberg says Ukraine will become a member of the alliance in the "long-term", but for now it needs to remain independent in the face of Russia's invasion.
Ukraine has sought to join the US-led military alliance for years.
After Russia's invasion of the country, President Volodymyr Zelensky asked for that request to be fast-tracked.
Ukraine also applied for EU membership days after Russia invaded, and gained candidate status in June.
"Nato allies have agreed that Ukraine will become a member of our alliance, but at the same time that is a long-term perspective," Mr Stoltenberg told reporters during a visit to Finland's capital, Helsinki.
"What is at issue now is that Ukraine can prevail as a sovereign independent nation."
Ukraine, for years, has sought to join the military alliance between the US, Canada and 28 European countries, something President Vladimir Putin has described as a security threat for Russia.
Mr Zelensky has pushed for fast-track accession, but it is unclear whether full membership is something the alliance members will seriously consider even after the war is over, despite pledges of support.
When the war ends, "we need to ensure that history doesn't repeat itself," Stoltenberg said, during 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," he is quoted as saying by the AFP news agency.
On Tuesday, Finland's parliament is due to start debating a bill to speed up the country's bid to join Nato, with a vote expected by Wednesday.
Following Russia's invasion of Ukraine, both Finland and Sweden decided they wanted to join the alliance as soon as possible, after being neutral for years.
Only Turkey and Hungary have yet to ratify the Finnish and Swedish bids to join the defensive bloc.
Turkey's government says both Sweden and Finland are harbouring Turkish citizens who it says are "terrorists" and is demanding they be extradited.