By Jonathan Landay and Andrew Gray

NATO members are racing to complete a plan to provide long-term support to Ukraine, but are wrestling with how best to assure the country's security until it can join the military alliance, according to U.S. and European officials.

With four weeks to go until a NATO summit in Vilnius that is expected to approve the plan, there is agreement that Ukraine cannot join the alliance while fighting is still underway against Russian forces, a position accepted in early June by President Volodymyr Zelenskiy after months of pleading for speedy admission.

Alliance members are close to agreeing incremental steps to strengthen ties with Ukraine, including upgrading how NATO and Kyiv cooperate and a multi-year program to help Ukraine bring its security forces to NATO operational and technical standards, according to officials.

The allies have yet to resolve differences over how to address Ukraine's desire for membership, which has been governed by a vague 2008 declaration that it will join the North Atlantic Treaty Organization without setting out how or when.

U.S. ambassador to NATO Julianne Smith told reporters on Wednesday that members are still discussing how to respond to the Kyiv government's membership aspirations.

"There's a rich conversation going on across the alliance with a whole array of views," said Smith.

A senior alliance source, speaking on condition of anonymity, said there is "a hard search on to find a mechanism that brings Ukraine closer to NATO without taking them into NATO."

Western governments such as the U.S. and Germany are wary of moves they fear could take the alliance closer to entering an active war with Russia, which has long seen NATO's expansion into eastern Europe as evidence of Western hostility.

Asked on June 2 about Ukraine's aspirations to join NATO, Kremlin spokesman Dmitry Peskov said it "would be a potential problem for many, many years."

Russian President Vladimir Putin sent his forces into Ukraine in February last year saying Russian security had to be protected. Few military analysts expect Ukraine's just-launched counteroffensive to bring the grinding conflict to a quick end - instead, many predict years of fighting.

Over that time, popular support for defending Ukraine in the West might fade and the 2024 U.S. election could yield an administration less willing to spend money on the war.

A CLEAR PATHWAY TO NATO?

Hanging over the deliberations is the question of whether alliance members can show unity by forging agreements ahead of the July 11-12 summit in the Lithuanian capital. Failing to do so would hand Putin a political and propaganda coup.

"Nobody wants to take a risk of disunity being displayed openly," said a senior Eastern European diplomat.

To reassure the Ukrainians, Poland and some other Eastern European governments have called on NATO to outline clear steps to eventual membership, and favor accelerated moves in that direction.

Others, particularly the United States and Germany, have been reluctant to embrace this idea, according to diplomats.

But all agree on the need to further boost Ukraine's security between now and the day it joins NATO.

"We must ensure that when this war ends, there are credible arrangements in place for Ukraine's security, so that history cannot repeat itself," NATO Secretary-General Jens Stoltenberg said on Wednesday.

Stoltenberg said these would include arrangements between Ukraine and a number of NATO allies. The precise nature of those arrangements is the subject of intense discussion.

Some leaders, such as Zelenskiy and French President Emmanuel Macron, have called for Ukraine to receive "security guarantees."

U.S. officials prefer the softer term "security commitments". They declined to define what those commitments would be, but said they were working on a mechanism that would allow individual countries to provide long-term military aid to Kyiv.

"What you will see as Vilnius approaches are increased discussions about what that mechanism could look like with the support of many of our allies and partners," a U.S. National Security Council spokesperson said.

Diplomats and officials said options under discussion include continued supplies of advanced weapons, ammunition and equipment, which has already amounted to tens of billions of dollars.

Some suggested loosely basing this on U.S. arrangements with Israel, whereby NATO states would offer fixed bilateral military assistance for a long period of time.

Gabrielle Tarini, co-author of a new RAND Corporation report on Ukraine reconstruction, said that until Ukraine can join NATO the alliance needs to explore such measures.

"Finding an approach that will be strong enough to deter Russian re-attack, but that does not necessarily provoke Russia will be the key here for security arrangements," she said.

Smaller steps are also in the works.

Stoltenberg said he expects the NATO-Ukraine Commission, a forum for cooperation, to be upgraded to become a NATO-Ukraine Council, where Kyiv would be accepted as an equal partner.

NATO will bolster a program of non-lethal aid for Ukraine's security forces to help them transition from Soviet-era to NATO standards, he said.

(Reporting by Jonathan Landay and Andrew Gray; Additional reporting by Humeyra Pamuk and Andrea Shalal in Washington and John Irish in Paris; Editing by Don Durfee and Grant McCool)