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. Military spending by member nations lags behind long-standing goals. 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.

Perhaps the most difficult questions are over how Ukraine should be eased into NATO. Some maintain admitting Ukraine would fulfill a promise made years ago and be a necessary step to deter Russian aggression in Eastern Europe. Others fear 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," President Joe Biden told CNN in an interview airing Sunday. He said joining NATO requires countries to “meet all the qualifications, from democratization to a whole range of other issues.”

He said the United States should provide long-term security assistance to Ukraine — "the capacity to defend themselves" — as it does with Israel.

Bickering among friends is not uncommon, and the current catalogue of disputes pales in comparison with past fears that Donald Trump would turn his back on the alliance during his presidency. But the current challenges come at a moment when 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, U.S. 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,” Lute said. “You don’t want to present any gaps or seams.”

By some measures, the war in Ukraine has reinvigorated NATO, which was created at the beginning of the Cold War as a bulwark against Moscow. NATO members have poured military hardware into Ukraine to help with its 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 Republican leader Mitch McConnell of Kentucky told 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 defense policy as well as increase in military spending in other countries.

The latest test of NATO solidarity came Friday with what Biden said was a "difficult decision” to provide cluster munitions to Ukraine. More than two-thirds of alliance members have banned the weapon because it has a track record for causing many civilian casualties. The U.S., Russia and Ukraine are not among the more than 120 countries that have not signed a convention outlawing the use of the bombs.

As for Ukraine's possible entry into NATO, the alliance said in 2008 that Kyiv eventually would become a member. Since then, little action has been taken toward that goal. Putin occupied parts of Ukraine in 2014 and then tried to capture the capital in 2022 with his invasion.

“A gray zone is a green light for Putin,” said Daniel Fried, a former U.S. ambassador to Poland who is now a distinguished fellow at the Atlantic Council.

Ukraine's president, Volodymyr Zelenskyy, called for a unified signal from NATO on Ukraine and for his country to join the alliance.

“It would be an important message to say that NATO is not afraid of Russia,” Zelenskyy said through a translator in an ABC interview, when asked whether he would come to Vilnius. “Ukraine should get clear security guarantees while it is not in NATO. And that is a very important point. Only under these conditions our meeting would be meaningful. Otherwise, it’s just another politics.”

The U.S. and Germany insist that the focus should be on supplying weapons and ammunition to Ukraine, rather than taking the more provocative step of extending a formal invitation to join NATO. Countries on NATO's Eastern flank — Estonia, Latvia, Lithuania and Poland — want firmer assurances on future membership.

NATO could decide 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 to Sweden’s attempts to join NATO alongside neighbor Finland.

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

Sweden recently changed its anti-terrorism legislation and lifted an arms embargo on Turkey. But a man burned a Quran outside a mosque in Stockholm last week, and Erdogan signaled that this would pose another hurdle. He equated “those who permitted the crime” to those who perpetrated it.

Turkey and the U.S. are also at an impasse over the sale of F-16 fighter jets. Erdogan wants the upgraded planes, but Biden says Sweden’s NATO membership has to be dealt with first. McConnell said in the AP interview that he supports the sale of the fighter jets to Turkey “provided that the membership of Sweden is settled.”

It’s not the first time that Erdogan has sought to use 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 Program at the Center for Strategic and International Studies, said there's growing frustration among allies toward Erdogan, building on concerns about his ties to Putin, democratic backsliding and sanctions evasion.

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

Hungary's prime minister, Viktor Orban, is also delaying his country's approval of Sweden’s membership. In response, Idaho Sen. Jim Risch, the top Republican on the Senate Foreign Relations Committee, is blocking a $735 million U.S. 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,” Risch said. “I’m just sick and tired of it."

But he 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 difficult issue is off the summit agenda. Rather than seek consensus on a new NATO leader, members agreed to extend the tenure of Jens Stoltenberg, who's held the job since 2014, for a year. It's his fourth extension.

Most members wanted a woman to be the next secretary-general, and Danish Prime Minister Mette Frederiksen had been considered a favorite. But Poland insisted on a candidate from the Baltic states because there had already been two Nordic secretaries general in a row. (Stoltenberg was a Norwegian prime minister and Rasmussen was a Danish prime minister.)

Others are skeptical of accepting a nominee from the Baltics, whose leaders tend to be more provocative in their approach to Russia, including supporting Ukraine's desire to rapidly join NATO.

More disagreements loom over NATO's updated plans for countering any invasion that Russia might launch on allied territory.