OSLO — The Biden administration and its NATO allies are developing plans for securing ongoing military support to Ukraine beyond the country’s current offensive, hoping that long-term security pacts will create a strong deterrent against future Russian aggression and potentially alter the battlefield calculus of President Vladimir Putin.

The evolving proposals represent an alternative to granting Ukraine admission into NATO in the near term, a prospect that has divided member states between those who support Kyiv’s request for immediate entry into the Western military alliance and those who fear it could plunge the bloc into direct conflict with Russia.

Officials from NATO nations, many of whom who spoke on the condition of anonymity to describe internal planning, described proposals for bilateral or multilateral agreements with Ukraine which they characterized as mutual defense pacts or security memorandums along the lines of those that have channeled billions of dollars a year in U.S. military aid to American ally Israel.

Following a months-long effort to secure donations of Western tanks, missiles, air defenses and other equipment that Ukrainian forces need for their planned operation — which some U.S. officials say is already underway — the Biden administration’s focus is now “how do we maintain that strength after the counteroffensive?” a senior U.S. official said.

“What we really want to do is send an unambiguous signal to Putin that regardless of what happens in the West in the coming couple of years, the West will remain supportive of Ukraine’s defense for as long as it takes,” the official said.

NATO Secretary General Jens Stoltenberg, speaking after talks among NATO foreign ministers in the Norwegian capital on Thursday, said member states strongly supported Ukraine’s eventual inclusion in NATO. But despite intensive advocacy by the government of Ukrainian President Volodymyr Zelensky, the bloc is not expected to issue a formal membership invitation at a summit of alliance leaders in Vilnius, Lithuania, next month.

Stoltenberg said Ukraine’s supporters must not only continue to provide arms needed to win the war, but also chart a post-conflict course. “When it ends, we need to ensure that we have the frameworks in place to ensure that this is not a pause in the pattern of Russian aggressive actions,” he said.

Officials have said the proposed agreements would involve locking in multiyear commitments to provide funding and weapons and potentially formal promises to come to one another’s aid if attacked. They would also serve to insulate Ukraine from major political shifts in the West or intensified calls to curtail aid, like those some U.S. Republicans have made.

U.S. officials for months have referred to a “porcupine model” that would arm Ukraine and make it capable of repelling Russian attacks, like Israel has attempted to do over decades as a deterrent to surrounding Arab states and Iran. Crucial in Israel’s strategy has been a series of 10-year memorandums with the United States that commit what amounts to more than $3 billion a year in security aid. The most recent memorandum was finalized in 2016.

The United States has provided Ukraine with nearly $40 billion in security aid since the war began.

Secretary of State Antony Blinken, speaking in Oslo, said Ukraine’s supporters were working to maximize its ability to recapture territory this year.

“We’re also focused on helping Ukraine build up its medium- to long-term deterrence and defense capacity, so that if and when the current aggression settles, Ukraine has the full capacity to deter and, if necessary, defend against future aggression,” he told reporters, suggesting details would emerge in coming weeks. Blinken is scheduled to give a speech in Finland on Friday about the U.S. outlook nearly a year and a half into the war.

According to European Commission President Ursula von der Leyen, a patchwork of security agreements could provide Ukraine “deterrence by denial.”

“What is important is that, at the end, there is clarity. Clarity that Ukraine’s friends will be there for the long haul for Ukraine’s security,” she said this week at a conference in Bratislava, Slovakia.

French President Emmanuel Macron issued his own call for security guarantees in Bratislava. “We have to build something between the security provided to Israel and full-fledged membership,” he said.

The timeline for such arrangements and their details — including, importantly, how they would work given Russia’s control of roughly a fifth of Ukrainian territory — remains unclear. U.S. officials said the deals, if they materialize as planned, were unlikely to be cemented before the Vilnius summit.

Alexander Vershbow, a former deputy NATO secretary general who is now a fellow at the Atlantic Council, said the goal would be to arm Ukraine “to the teeth.”

“This could gain further traction as alliance policy at the NATO summit in Vilnius, even though membership would be the ultimate destination,” he said.

Eric Ciaramella, a former White House official who is now a fellow at the Carnegie Endowment for International Peace, said a security framework would show Putin that the West’s support will not fade as war fatigue sets in or as political leadership changes, an idea which appears to be a key plank in Russia’s strategy.

“We need to start laying out a more sustainable and predictable framework that takes a lot of what we’ve been doing so far and codifies it . . . in a way that shows Putin that we mean it when we say we’re in it to support Ukraine for the long haul,” Ciaramella said.

While President Biden has had strong support in Congress for his campaign of massive security aid to Ukraine, a minority of Republicans have argued against continuing high levels of assistance to Ukraine.

Such deals would fill a void while haggling continues over Ukraine’s NATO membership, a process that could take years and is vulnerable to veto by members such as Hungary and Turkey, which have more friendly relations with Moscow.

They would also provide the arms industries in Europe and the United States the funds and demand needed to increase production of vital weaponry, which remains a major challenge 15 months into the war.

Some former officials cautioned that such agreements should not be seen as a substitute for the formal security guarantees enshrined in NATO’s Article 5, the bloc’s mutual defense clause. They note that Ukraine, unlike Israel, faces an adversary that is the world’s largest nuclear power.

Andrea Kendall-Taylor, a former deputy national intelligence officer for Russia and Eurasia at the National Intelligence Council, said anything short of a security guarantee would keep Ukraine in what she said was a “gray zone” the former Soviet republic has occupied since the end of the Cold War.

“The porcupine plan should not be a replacement for Article 5,” she said. “It has to be both.”

Gen. Richard Barrons, former commander of the British military’s Joint Forces Command, cautioned that a highly armed Ukraine, without the constraints of NATO membership, could have other, more problematic outcomes for the West.

“A properly rearmed Ukraine would be a strong deterrent to Russia, but it would be possibly tempted to have its own adventure,” Barrons said. “The best answer from a Ukrainian perspective, if not necessarily from a Western perspective, is to have Ukraine in NATO, because then the guarantees are clear and difficult to dodge, and also Ukraine has to subscribe to NATO ambition and policy, so the adventurism is less.”

Planning occurs as officials in some NATO nations attempt to temper expectations about what Ukraine’s long-awaited offensive will look like and what it will achieve. Leaked American defense documents show that the U.S. government, in an analysis early this year, said it would be difficult for Ukraine to make anything more than modest gains because of personnel and equipment challenges.

Announcing such deals in the coming months could also counteract a failure by Ukrainian forces to recapture large amounts of territory this year. As the operation gets off to a gradual start, the senior U.S. official suggested that even modest gains could have an impact on Putin’s calculus in Ukraine.

“At the end of the day, they’re going to again show Putin that his claims that these territories have been illegally annexed by Russia are false. He doesn’t control all of them now, and he’s going to control less of them by the fall,” he said. “That ultimately will be a strong signal to Russian elites, and to President Putin, that the momentum in this over the long term is not in his favor.”

Belton reported from London and Washington. Rauhala reported from Brussels.