As world leaders prepare to gather this week for a much-anticipated NATO summit in Vilnius, Lithuania, the alliance's unity may be newly tested as divisions over the type of assurances made to Ukraine regarding its future membership become starker. 

More than 500 days into a grinding war launched by Russia in February 2021, Ukraine has pressed for a detailed path toward membership into the alliance that some member countries appear reluctant to provide.

The impasse has prompted concerns in Kyiv and among some eastern European NATO members that the Vilnius summit – touted for weeks by the Biden administration as a show of enduring support – may result again in ambiguous commitments reminiscent of those made in Bucharest in 2008.

It was there that NATO members told Ukraine and fellow candidate Georgia they "will become members" of the alliance, but – despite public pressure from then-U.S. President George W. Bush – declined to offer them Membership Action Plans (MAP) that would have laid out a detailed roadmap.

Later that year, Russia invaded Georgia, and six years later in 2014, seized Crimea. 

Ukrainian President Volodymyr Zelenskyy and some of his staunchest allies in eastern Europe are now arguing that clearly defined commitments from NATO are essential to boosting Ukrainian fighters' morale as they wage a grueling counteroffensive and to deterring Russian President Vladimir Putin, who has banked on waning interest in support for Ukraine from Western capitals.

"We are getting ready for NATO membership," said Ukrainian ambassador to the U.S. Oksana Markarova in an interview with CBS's "Face the Nation" on Sunday.

Markarova said that in 2008, the "open door policy" toward Ukraine had been adopted, and now "we want not only for the door to be open, we want to be invited to come in." 

"The Eastern Europeans and particularly the hosts have been very forward-leaning, but they've already been told by other allies that they can't be that forward-leaning," said one Western diplomat, speaking on the condition of anonymity to discuss sensitive negotiations.

While there have been "intensive discussions" going into the summit on how to update the commitments made in Bucharest, the official said, "it wouldn't be the first multilateral meeting that fell back on something less than crystal clear."

"Fundamentally, the message to the rest of the world has to be that the direction of travel [for Ukraine] hasn't changed," the Western diplomat said.

The Biden administration has already said Ukraine, with an open border conflict, would not be offered membership at Vilnius.  

"I don't think it's ready for membership in NATO," President Biden said about Ukraine in an interview with CNN on Friday. 

However, Mr. Biden then added, "I don't think there is unanimity in NATO about whether or not to bring Ukraine into the NATO family now, at this moment, in the middle of a war." 

Sen. Chris Coons, a member of the Senate Foreign Relations Committee, told "Face the Nation" moderator Margaret Brennan that "there has to be a security guarantee for Ukraine going forward," but that a firm assurance on NATO membership is "a decision for 31 NATO members to make." 

"They'll make real progress on sustaining or critical support in the middle of this counter offensive," said Coons, "but I don't think they'll leave Vilnius with a specific timeline." 

Jake Sullivan, Mr. Biden's National Security Advisor, expressed a similar message at the White House press briefing on Friday.

"The president also has been clear that we are going to support Ukraine for as long as it takes and provide them an exceptional quantity of arms and capabilities, both from ourselves and facilitating those from allies and partners," said Sullivan, "but that we are not seeking to start World War III."

NATO Secretary General Jens Stoltenberg said Friday the alliance would agree to a "multi-year program of assistance" to Ukraine and establish a NATO-Ukraine Council to "upgrade our political ties."

"I expect allied leaders will reaffirm that Ukraine will become a member of NATO," Stoltenberg added, "and unite on how to bring Ukraine closer to its goal.

In the meantime, Mr. Biden is still trying to usher in Sweden as a new NATO member. Coons, a fellow Democrat from Delaware and a close Biden ally, said Sunday that Mr. Biden  has been personally engaged in trying to persuade reluctant NATO members like Greece to let in Sweden. 

"We have 31 members of NATO today, there should be 32. Adding Finland and Sweden to NATO is a strategic defeat for Putin. It means that no matter the outcome on the ground in Ukraine, he has failed in his objective to divide and weaken NATO, because of President Biden's leadership. NATO is the strongest it's ever been," said Coons. "My hunch is they'll make real progress on Sweden accession."

In:
• Ukraine
• NATO

Sophia Barkoff is a broadcast associate with CBS News' "Face the Nation."