"To become a member in the midst of a war is not on the agenda," he said at an event organized by the German Marshall Fund of The United States in Brussels. "The issue is what happens when the war ends."

In September, President Volodymyr Zelenskiy announced a bid for fast-track membership of NATO after Russian President Vladimir Putin proclaimed four partially occupied regions of Ukraine as annexed Russian land.

NATO allies have not acceded to Zelenskiy's request, with Western governments wary of moves that they fear could take NATO closer to entering an active war with Russia.

However, both Kyiv and some of its closest allies in eastern Europe have been pushing for NATO to at least take concrete steps to bring Ukraine closer to membership at the alliance's summit in Vilnius in July.

"It is time for the alliance to stop making excuses and start the process that leads to Ukraine's eventual accession," Ukrainian Foreign Minister Dmytro Kuleba wrote in an op-ed for Foreign Affairs in April. "What we need is a clear written statement from the allies laying out a path to accession."

Latvian Prime Minister Krisjanis Karins said Russia would start a conflict all over again unless Ukraine was allowed to join NATO after the end of the war.

"To have ... lasting peace, we will need Ukraine, (an) independent, free and liberated Ukraine, as a part of the NATO alliance," he said after meeting Stoltenberg.

In a rare visit to Kyiv in April, Stoltenberg underscored that Ukraine's future lies in NATO, a strong reaffirmation of a 15 year-old decision. But he did not give a timeline.

NATO agreed at its 2008 summit in Bucharest that Ukraine will eventually become a member of the alliance.

However, leaders have since stopped short of taking any steps such as giving Kyiv a membership action plan that would lay out a timetable for bringing the country closer to NATO.

At the Brussels event, Stoltenberg acknowledged there were differences among NATO members over how to address Kyiv's membership ambitions.

"There are different views in the alliance and, of course, the only way to make decisions in NATO is by consensus. There are consultations going on now," he said.

"No one is able to tell you exactly what will be the final decision of the Vilnius summit on this issue."