President Biden’s high-profile diplomatic trip to the NATO summit in Europe has exposed some cracks in the U.S.’s relationship with Ukrainian President Volodymyr Zelensky, who said a lack of urgency to include his country in the alliance would only allow Russia to “continue its terror.”

Biden has hit the brakes on NATO accepting Ukraine into the alliance, after the war-torn country has been vying for a membership. Biden took to Vilnius, Lithuania, to assure other world leaders that a country engaged in a war with Russia wouldn’t be accepted as a member — citing the mutual defense agreement that a war with a NATO ally is a war with all.

That point was reiterated later by national security spokesman John Kirby, who said the inclusion of Ukraine “isn’t likely” while acknowledging: “Yes, there are frustrations.”

Biden’s comments on Ukraine not being ready to join NATO were some of his clearest yet on the matter, and one in which other alliance leaders agree, much to Zelensky’s ire.

“There is a lot of question marks, not only in the Ukrainian government but also among Ukrainians, about why it is the United States, which has been so supportive of Ukraine in the past … [being] the ones who are dragging on any firmer commitment for Ukraine’s NATO perspective,” said Liana Fix, a fellow for Europe at the Council on Foreign Relations.

The tone Wednesday was markedly more conciliatory when Biden met with Zelensky on the sidelines of the summit. But Biden made sure to reiterate his stance that Ukraine isn’t ready for a NATO membership. 

“I think until the war is over,” Biden said on when Ukraine can join the alliance, adding, “The one thing Zelensky understands now is that whether or not he’s in NATO now is not relevant as long as he has the commitments that … guarantee his security.”

But one day earlier, Zelensky lambasted the alliance for not setting a timeline for his country to join NATO, saying, “It seems there is no readiness neither to invite Ukraine to NATO nor to make it a member of the Alliance.” 

Zelensky shifted his rhetoric Wednesday, when NATO leaders said that a membership action plan, which gives advice for a country seeking to join, would not be required for Ukraine’s future membership into NATO. But, he has continued to take issue with the wording that Ukraine needs to meet “conditions” to be invited to NATO.

“The absolute majority of our people expect specifics about these conditions. We perceive them as security conditions. We understand that Ukraine cannot become a member of NATO while the war is ongoing. But then it will be our common strength when Ukraine joins the Alliance,” the Ukrainian president wrote on Twitter.

The Biden-Zelensky relationship hasn’t always been perfect. Biden last year reportedly lost his temper with the Ukrainian president, asking him to show some gratitude for the assistance the U.S. has provided, while Zelensky was asking for more.

Though frustrations with Zelensky have bubbled up before, they have not been so apparent on the world stage as they were this week in Europe. Biden and Zelensky have demonstrated a strong bond since the start of Ukraine’s 18-month-old war, but the cracks exposed at the summit marked a change in tone compared late last year, when Zelensky traveled to Washington, his first time leaving his country for the first time since the Russian invasion.

Biden, in turn, made a surprise and historic visit to Kyiv in February to mark the anniversary of the war beginning. The two visits elevated Biden’s role as the leader of NATO when it comes to the response to Russia and support for Ukraine.

But just days before he embarked on the Europe trip, Biden said in a televised interview that Ukraine wasn’t ready to join NATO, setting off a series of alliance leaders falling behind the U.S. president.

The outburst by Zelensky was squashed relatively quickly this week. The Ukrainian president complimented Biden following their one-on-one meeting Wednesday and even acknowledged the damaging political implications of approving controversial cluster bombs to Kyiv.

“It feels like the Ukrainians realize that they have overstepped a bit, that they don’t have the power to do that — to damage the relationship that they have with the US. And they’re sort of slowly walking it back. I would say it’s not really a rift in the relationship … but it’s a testing of Ukraine,” Fix said.

Luke Coffey, senior fellow at Hudson Institute and a former adviser at the United Kingdom Ministry of Defence, said that while it’s not realistic that a country at war would be brought into NATO, Biden backlash is largely due to messaging.

“I think there’s a problem with how all this stuff is being communicated. Whenever President Biden says that Ukraine isn’t ready to join NATO, what I suspect he means — and what many other NATO leaders mean — is that as long as Ukraine is involved in a war with Russia, it can’t join the alliance,” Coffey said.

“I think if these things were worded a bit differently, and more delicately, you could get the same message across without all of the drama that we’re seeing right now,” he added.

Since Zelensky’s criticism about a lack of a timeline for joining the alliance, the White House has doubled down on the acknowledgment that Ukraine will eventually be a part of NATO. 

“We joined with allies yesterday in agreeing to a very strong, positive message.  We reaffirmed that Ukraine will become a member of the NATO Alliance,” Amanda Sloat, National Security Council senior director for Europe, said Wednesday.

Biden has had to keep support up for Ukraine at home and avoid seeing a drop in enthusiasm for assisting the country, while some Republican lawmakers have suggested the U.S. can’t provide a blank check. But a Reuters/Ipsos poll last month found 65 percent of respondents want the U.S. to continue arming Ukraine, compared to 48 percent who said the same thing back in May. 

Fix argued that the personal relationship between the two presidents has remained strong through those issues and predicted it will be able to withstand the latest bumps.

“I think this is kind of a personal relationship that is between Biden and Zelensky that Zelensky has gone to Washington, Biden to Kyiv,” she said. “I mean, that really established such a strong relationship between the two of them, and Biden has been really the one who’s having Zelensky’s back.”