As the Russian invasion of Ukraine continues with no end in sight, Nato’s much-celebrated unity faces fresh strains when leaders gather for their annual summit this week in Vilnius, Lithuania.

The world’s biggest security alliance is struggling to reach an agreement on admitting Sweden as its 32nd member.

And there is controversy about the US’s announcement on Friday that it will provide Ukraine with cluster munitions.

Military spending by member nations still lags behind longstanding goals. And an inability to compromise over who should serve as Nato’s next leader forced an extension of the current secretary general’s term for an extra year.

There are questions over how Ukraine should be eased into the alliance. Some maintain admitting Ukraine to Nato would be the fulfilment of a promise made years ago and a necessary step to deter Russian aggression in Eastern Europe.

Others are fearful it would be seen as a provocation that could spiral into an even wider conflict.

“I don’t think it’s ready for membership in Nato,” US president Joe Biden told CNN in an interview broadcast on Sunday.

He said joining Nato required countries to “meet all the qualifications, from democratisation to a whole range of other issues”.

Mr Biden said the US should provide long-term security assistance to Ukraine — “the capacity to defend themselves” — as it did with Israel.

The challenges come at a moment when Mr Biden and his counterparts are heavily invested in demonstrating harmony among members.

“Any fissure, any lack of solidarity provides an opportunity for those who would oppose the alliance,” said Douglas Lute, who served as US ambassador to Nato under President Barack Obama.

Russian President Vladimir Putin is eager to exploit divisions as he struggles to gain ground in Ukraine and faces political challenges at home, including the aftermath of a brief revolt by the Wagner mercenary group.

“You don’t want to present any openings,” Mr Lute said. “You don’t want to present any gaps or seams.”

Members of the alliance have poured military hardware into Ukraine to help with its ongoing counteroffensive, and Finland ended a history of nonalignment to become Nato’s 31st member.

“I think it’s appropriate to look at all the success,” Senate Minority Leader Mitch McConnell said in an interview with The Associated Press. “So I think the invasion has strengthened Nato — exactly the opposite of what Putin anticipated.”

He noted Germany’s shift toward a more robust defence policy as well as other countries’ increase in military spending.

The US announced on Friday it will provide Ukraine with the controversial cluster munitions.

Such a bomb poses a higher risk of civilian harm as it opens in the air releasing smaller “bomblets” across a wide area, hitting multiple targets simultaneously. Ukraine has promised to use them carefully.

In a statement on Saturday evening, Italian Premier Giorgia Meloni — one of Western Europe’s staunchest backers of Ukraine in the war — reiterated her country’s condemnation of the Russian aggression but called for the “universal application of the principles” of the international convention banning the production, transfer and stockpiling of cluster munitions.

Spain’s defence minister Margarita Robles said while her country respects the US decision, it did not agree with it.

“No to cluster bombs and yes to helping in the legitimate defence of Ukraine, which we understand should be carried out without that type of bomb,” she said.

Canada and the UK also voiced concerns while Germany, which has signed the ban treaty, said it will not provide the bombs to Ukraine, but expressed understanding for the American position.

UN deputy spokesman Farhan Haq said Secretary-General Antonio Guterres “wants countries to abide by the terms of that convention and so as a result, of course, he does not want there to be continued use of cluster munitions on the battlefield”.

But the ongoing war has allowed other challenges to fester or bubble to the surface.

In particular, Nato leaders said in 2008 that Ukraine would eventually become a member, but little action has been taken toward that goal. Mr Putin occupied parts of the country in 2014 and then attempted to capture Kyiv in 2022.

“A grey zone is a green light for Putin,” said Daniel Fried, a former US ambassador to Poland, and now a fellow at the Atlantic Council.

The US and Germany insist the focus should be supplying weapons and ammunition to help Ukraine win the current conflict, rather than taking the more provocative step of extending a formal invitation to join Nato.

However, countries on Nato’s Eastern flank — Estonia, Latvia, Lithuania and Poland — want firmer assurances on future membership.

Ukrainian President Volodymyr Zelensky is pushing for that as well. During a visit to Prague on Thursday, he said the “ideal” result of the Vilnius summit would be an invitation for his country to join the alliance.

Jake Sullivan, Mr Biden’s national security adviser, described the summit as “an important moment on that pathway toward membership” and that allies need to “discuss the reforms that are still necessary for Ukraine to come up to Nato standards.”

Nato could use the occasion to elevate its relationship with Ukraine, creating what would be known as the Nato-Ukraine Council and giving Kyiv a seat at the table for consultations.

Also in the spotlight in Vilnius will be Turkish President Recep Tayyip Erdogan, the main obstacle blocking Sweden’s attempts to join Nato alongside its neighbour Finland.

Mr Erdogan accuses Sweden of being too lenient on anti-Islamic demonstrations and militant Kurdish groups that have waged a decades-long insurgency in Turkey.

Sweden recently changed its anti-terrorism legislation and lifted an arms embargo on Turkey. However, a man burned a Quran outside a mosque in Stockholm last week, and Mr Erdogan signalled that this would pose another obstacle.

He equated “those who permitted the crime” to those who perpetrated it.

Turkey and the US are also at an impasse over the sale of F-16 fighter jets. Mr Erdogan wants the upgraded planes, but Mr Biden says that Sweden’s Nato membership has to be dealt with first.

Mr Sullivan said the US is confident that Sweden will join Nato “in the not-too-distant future,” but it is unclear if the matter will be resolved during the summit.

It is not the first time that Mr Erdogan has used a Nato summit for Turkish gain. In 2009, he held up the nomination of Anders Fogh Rasmussen as secretary general but agreed to the move after securing some senior posts for Turkish officials at the alliance.

Max Bergmann, a former State Department official who leads the Europe Programme at the Centre for Strategic and International Studies, said there is growing frustration among allies toward Mr Erdogan, building on concerns about his ties to Mr Putin, democratic backsliding and sanctions evasion.

“They’ve tried playing nice,” Mr Bergmann said. “The question is whether it’s time to get much more confrontational.”

Hungarian Prime Minister Vitkor Orban is also delaying his country’s approval of Sweden’s membership. In response, Senator Jim Risch, the top Republican on the Senate Foreign Relations Committee, is blocking a 735 million dollar (£572 million) US arms sale to Hungary.

“We don’t want members who aren’t interested in doing everything possible to strengthen the alliance rather than the pursuit of their own or individual interests,” he said. “I’m just sick and tired of it.”

However, Mr Risch rejected the idea that these disagreements are a sign of weakness within Nato.

“These are kinds of things that always arise in an alliance,” he said. “The fact that we’ve been able to deal with them and will continue to deal with them proves that this is the most successful and strongest military alliance in the history of the world.”

At least one potentially flammable item has been taken off the summit agenda. Rather than seek consensus on a new Nato leader, members agreed to extend Jens Stoltenberg’s tenure for a year. He has had the job since 2014, and it is the fourth time that his time in office has been extended.

More disagreements loom over Nato’s updated plans for countering any invasion that Russia might launch on allied territory. It is the biggest revision since the Cold War, and Skip Davis, a former Nato official who is now a senior fellow at the Centre for European Policy Analysis, said it could involve “lots of arm wrestling and card trading”.

“That’s an issue that will cause tension and dissent, and that’s not what the Vilnius summit is all about,” he said.