NATO leaders agreed Tuesday to allow Ukraine to join “when allies agree and conditions are met,” the head of the military alliance said, hours after President Volodymyr Zelenskyy blasted the organization’s failure to set a timetable for his country as “absurd.”

“We reaffirmed Ukraine will become a member of NATO and agreed to remove the requirement for a membership action plan,” NATO Secretary-General Jens Stoltenberg told reporters, referring to a key step in joining the alliance.

Also read: NATO summit boosted by Turkey's decision to end opposition to Sweden's bid to join alliance

“This will change Ukraine’s membership path from a two-step path to a one-step path,” he said.

Although many NATO members have funneled arms and ammunition to Zelenskyy’s forces, there is no consensus among the 31 allies for admitting Ukraine into NATO’s ranks. Instead, the alliance leaders decided to remove obstacles on Ukraine’s membership path so that it can join more quickly once the war with Russia is over.

Zelenskyy pushed back sharply against the decision.

Also read: Erdogan says no change in Turkey's stance on Sweden's NATO membership

“It’s unprecedented and absurd when a time frame is set neither for the invitation nor for Ukraine’s membership,” Zelenskyy tweeted as he headed to the summit. “While at the same time, vague wording about ‘conditions’ is added even for inviting Ukraine. It seems there is no readiness to invite Ukraine to NATO or to make it a member of the Alliance."

Asked about Zelenskyy’s concerns, Stoltenberg said that the most important thing now is to ensure that his country wins the war, because “unless Ukraine prevails there is no membership to be discussed at all.”

The broadside from Zelenskyy could renew tensions at the summit shortly after it saw a burst of goodwill after Turkey agreed to advance Sweden's bid to join NATO. Allies hope to resolve the seesawing negotiations and create a clear path forward for the alliance and its support for Ukraine.

“We value our allies,” Zelenskyy wrote on Twitter, adding that “Ukraine also deserves respect.” He also said: “Uncertainty is weakness. And I will openly discuss this at the summit.”

Zelenskyy is expected to meet with U.S. President Joe Biden and other NATO leaders on Wednesday.

There have been sharp divisions within the alliance over Ukraine's desire to join NATO, which was promised back in 2008 even though few steps were taken toward that goal.

In addition, the Baltic states — including Lithuania, which is hosting the summit — have pushed for a strong show of support and a clear pathway toward membership for Ukraine.

However, the United States and Germany urged caution. Biden said last week that Ukraine was not ready to join. Members of NATO, he told CNN, need to “meet all the qualifications, from democratization to a whole range of other issues,” a nod toward longstanding concerns about governance and corruption in Kyiv.

Also read: Biden’s upcoming European trip is meant to boost NATO against Russia as the war in Ukraine drags on

In addition, some fear that bringing Ukraine into NATO would serve more as a provocation to Russia than as a deterrence against aggression.

White House national security adviser Jake Sullivan said allies were debating the “precise nature” of Ukraine’s pathway to membership. However, he promised that the summit would demonstrate how Russian President Vladimir Putin’s hopes for fractures within NATO will go unfulfilled.

The dispute over Ukraine stands in contrast to a hard-fought agreement to advance Sweden's membership. The deal was reached after days of intensive meetings, and it's poised to expand the alliance's strength in Northern Europe.

“Rumors of the death of NATO’s unity were greatly exaggerated," Sullivan told reporters triumphantly on Tuesday.

According to a joint statement issued when the deal was announced, Turkish President Recep Tayyip Erdogan will ask Turkey's parliament to approve Sweden joining NATO.

Hungarian Prime Minister Viktor Orban, another holdout, is expected to take a similar step. Hungary's foreign minister said Tuesday that his country's ratification of Sweden's NATO membership was now just a “technical matter.” Erdogan has not yet commented publicly.

The outcome is a victory as well for Biden, who has touted NATO's expansion as an example of how Russia's invasion of Ukraine has backfired on Moscow.

Finland has already become the 31st member of the alliance, and Sweden is on deck to become the 32nd. Both Nordic countries were historically nonaligned until the war increased fears of Russian aggression.

Because of the deal on Sweden's membership, "this summit is already historic before it has started,” Stoltenberg said.

Kremlin spokesman Dmitry Peskov told reporters that NATO's expansion is "one of the reasons that led to the current situation.”

"It looks like the Europeans don’t understand their mistake,” Peskov said. He warned against putting Ukraine on a fast track for NATO membership.

“Potentially it’s very dangerous for the European security, it carries very big risks," Peskov said.

Erdogan met with Biden on Tuesday evening but remained mum on the deal to advance Sweden’s membership in NATO.

Although Biden made a reference to “the agreement you reached yesterday,” Erdogan said nothing about it. It was a conspicuous omission from Erdogan, who has not commented on the issue publicly during the summit.

However, Erdogan appeared eager to develop his relationship with Biden. He said previous meetings were “mere warm-ups, but now we are initiating a new process.”

The Turkish president has been seeking advanced American fighter jets and a path toward membership in the European Union. The White House has expressed support for both, but publicly insisted that the issues were not related to Sweden's membership in NATO.

The Biden administration has backed Turkey’s desire to buy 40 new F-16s as well as modernization kits from the U.S. It's a move some in Congress, most notably Senate Foreign Relations Committee Chairman Bob Menendez, D-N.J., have opposed over Turkey blocking NATO membership for Sweden, its human rights record and other concerns.

In Washington, Menendez said he continued to have "reservations” on providing the fighter aircraft to Turkey. If the Biden administration could show that Turkey wouldn’t use the F-16s belligerently against other NATO members, particularly neighboring Greece, and meet other conditions, “then there may be a way forward,” Menendez told reporters.

Biden is on a five-day trip to Europe, with the NATO summit as its centerpiece.

The president spent Monday in the United Kingdom, meeting at Windsor Castle with King Charles III and in London with Prime Minister Rishi Sunak.

He met Tuesday with Lithuanian President Gitanas Nauseda, emphasizing his commitment to transatlantic cooperation, before he joined the NATO gathering.

“Nothing happens here that doesn’t affect us,” Biden told Nauseda. The White House said Nauseda presented Biden with the Order of Vytautas the Great, the highest award a Lithuanian president can bestow. Biden is the first U.S. president to receive it.

After the summit ends Wednesday, Biden will travel to Helsinki. On Thursday, he will celebrate Finland's recent entry into NATO and meet with Nordic leaders.