Media reports say Ukrainian President Volodymyr Zelenskyy says his country "deserves respect" from its allies as he arrives in the Lithuanian capital for the conference.
The Ukrainian president has criticized the "absurd" absence of a timetable for his country to join NATO as leaders met at a summit in Lithuania.
US President Joe Biden described the gathering as a "historic moment" and said Washington agreed with a proposal, yet to be released publicly, to outline a path for Ukraine's eventual membership of the alliance.
NATO agrees Ukraine will join alliance and sets out 'clear path' to membership. While Ukraine’s NATO allies reaffirmed their support for Kyiv’s bid for membership, they remained vague about the timing, according to CNN.
Zelenskyy, who was in Vilnius for the summit, reportedly expressed disappointment at how the negotiations were playing out.
"We value our allies," he wrote on Twitter but added that "Ukraine also deserves respect".
"It's unprecedented and absurd when timeframe is set neither for the invitation nor for Ukraine's membership," Zelenskyy wrote on Twitter.
He added: "Uncertainty is weakness. And I will openly discuss this at the summit."
Mr. Zelenskyy later addressed a crowd at a concert being held alongside the conference in Lithuania's capital, telling a crowd full of people waving Ukrainian flags that "NATO will make Ukraine safer and Ukraine will make NATO stronger".
Sky News reports that responding to Zelenskyy's comments, NATO Secretary-General Jens Stoltenberg said a timeline for Ukraine's membership in the alliance has not been set out as it is "conditions-based".
Speaking at a news conference yesterday afternoon, Stoltenberg reportedly said there has "never been a stronger message from NATO at any time".
The alliance chief said members had agreed a "substantive package" to move Ukraine closer and were sending a "strong political message with the language on membership".
"If you look at all membership processes there have not been timelines… they are conditions based, have always been," Mr. Stoltenberg told reporters in Vilnius.
