NATO Secretary General Jens Stoltenberg said he had put forward a package that included the removal of the requirement for a Membership Action Plan (MAP) - a list of political, economic and military goals that other eastern European nations had to meet before joining the alliance.

But Ukrainian President Volodymyr Zelenskiy, who is expected to attend the summit, wants a clear invitation in Vilnius to join the alliance after Russia's war on Ukraine ends, and security guarantees until that time.

NATO members in Eastern Europe, under Moscow's thumb for decades in the last century, have backed Ukraine's stance. But others, such as the United States and Germany, have been more cautious, wary of any move that they fear could draw NATO into a direct conflict with Russia, and potentially spark a global war.

Ukrainian Foreign Minister Dmytro Kuleba tweeted that there had been a consensus among allies to drop MAP, but added: "It is also the best moment to offer clarity on the invitation to Ukraine to become member."

Stoltenberg told a news conference there would be more meetings on Monday: "No final decision has been made but at the summit I am absolutely certain that we will have unity and a strong message on Ukraine."

President Vladimir Putin has cited NATO's expansion towards Russia's borders over the past two decades as a reason for his decision to send his armed forces into Ukraine on Feb. 24, 2022.

Assertions that "Ukraine's rightful place is in NATO" and that it will join "when conditions allow" are among the phrases being discussed, diplomats say, as they try to find wording acceptable to all NATO's 31 members.

At a summit in Bucharest in April 2008, after much wrangling, NATO declared that both Ukraine and Georgia would join the U.S.-led alliance - but gave them no plan for how to get there.

Ukraine's eastern allies demand that Vilnius go beyond the 2008 declaration, and the issue may again be left for leaders to resolve. Two European diplomats said positions remained entrenched and little progress had been made over the weekend.

TURKEY PLAYS EU CARD

Standing beside Stoltenberg, Lithuanian President Gitanas Nausėda said a prospect of membership was extremely important to Ukraine, which had been "heroically fighting the Russian monster for almost one and a half years".

"We must avoid Ukraine membership ... becoming a horizon," he said. "The more you walk towards it, the farther it is."

Ukraine's largest Western backers were also still finalising a joint framework to enable long-term security assurances for Kyiv from individual allies, but may wait until after summit to announce them - to make clear they do not come from NATO - European diplomats say.

Tuesday will also see further talks between NATO, Sweden and Turkey over Stockholm's membership. Ankara has been the main obstacle to Sweden's bid, which requires unanimous approval by all NATO members.

In an unexpected move, Turkish President Tayyip Erdogan said the European Union should open the way for Ankara's accession to the bloc before Turkey's parliament approves Sweden's bid to join the NATO military alliance.

Stoltenberg said Sweden had met previously agreed conditions and that it was still possible Ankara's reticence could be overcome during the summit, where Erdogan would hold face-to-face talks with U.S. President Joe Biden.

With the war in Ukraine casting its shadow, the summit will be guarded by Patriot missile batteries from Germany, fighter jets, and forces from 17 nations.