Ukrainian President Volodymyr Zelensky on Tuesday criticised NATO's "uncertainty" and "weakness" over his country's possible membership as the alliance's leaders met in Vilnius, saying this encouraged Russian aggression.

In a message on Twitter, Zelensky said he received "signals" that a "certain wording is being discussed" by NATO over issuing an invitation to Ukraine to become a member.

"It's unprecedented and absurd when (a) time frame is not set neither for the invitation nor for Ukraine's membership," Zelensky wrote.

"It seems there is no readiness neither to invite Ukraine to NATO nor to make it a member of the Alliance," he added.

He said that this was apparently to give NATO "a window of opportunity" for bargaining over Ukraine's membership in talks with Russia.

"And for Russia, this means motivation to continue its terror," Zelensky added.

"Uncertainty is weakness. And I will openly discuss this at the summit," he concluded, as he is expected to attend the two-day event