Joe Biden skipped a dinner with Nato leaders on Tuesday, instead returning to his hotel room in Lithuania.
A US official was reported as saying that the US .president skipped the dinner because he has four full days of official business ahead of him and is preparing for a major speech on Wednesday.
Mr Biden, who arrived in Europe on Sunday, sent Secretary of State Antony Blinken to the dinner in his place.
The gathered leaders have offered an invitation to Ukraine to join in the defence alliance, pending certain conditions.
Before Ukraine can join, all 31 member nations must agree with its inclusion among other conditions.
"We will be in a position to extend an invitation to Ukraine to join the Alliance when Allies agree and conditions are met,” Nato said in an official statement released on Tuesday.
Jens Stoltenberg, the the Secretary General of Nato, said the allies had reduced the bureaucratic hurdles Ukraine would need to clear in order to join the alliance.
"What we have agreed to is a very substantive package with many different elements that helps to move Ukraine closer to NATO,” Mr Stoltenberg said.
Ukraine President Volodymyr Zelensky expressed frustration that Nato did not provide a timeframe for Ukraine's inclusion in the organisation, calling it "unprecedented and absurd."
He further insisted that the lack of a timeframe provides Russia with a "window of opportunity" for Russia to include Ukraine's potential Nato membership in future negotiations.
"And for Russia, this means motivation to continue its terror," he said.
Mr Stoltenberg contended that there have never been timelines given to prospective member nations, saying inclusion is always condition based.
“If you look at other membership processes, there have not been timelines,” he said. “They are condition based — has always been.”
Despite his frustrations, Mr Zelensky attended a flag ceremony in Vilnius, Lithuania during which he said he had "embarked on a trip here with faith in decision, with faith in partners, with faith in a strong NATO."
He further said he trusted Nato would not "hesitate" or "waste time" moving toward including Ukraine in the alliance.
"NATO will give Ukraine security. Ukraine will make the alliance stronger," Mr Zelensky said.
