A public clash over Turkey’s insistence on purchasing a Russian anti-aircraft missile defense system threatens to fracture NATO, says a top diplomat from the alliance.

“I am worried about that,” Lithuanian foreign minister Linas Linkevičius told the Washington Examiner. “To lose a very important ally, it's not a success. Of course it's failure.”

Diplomats rarely talk openly about the possibility of a divorce between Turkey and the rest of the transatlantic alliance, but this week's summit in Washington marking NATO's 70th anniversary featured a public airing of grievances between Turkey and the United States. The chief controversy revolves around an arms deal that Western powers fear would give Russia precious insights into NATO military systems, and the technical dispute has broad geopolitical ramifications.

“It looks like a big, hot issue,” Linkevičius said. “It's a big deal.”

No member of NATO has ever left the alliance. A split from Turkey, which joined in 1952 and began hosting American nuclear weapons nine years later, would be especially consequential. Another top diplomat cautioned that NATO allies are not talking about any formal break with Turkey.

“Turkey has the second-largest army of NATO,” Hungarian Foreign Minister Péter Szijjártó told the Washington Examiner on the sidelines of the summit. “I have never heard from anyone any kind of intention to break this cooperation. And that would be not rational at all.”

The two sides appear to be deadlocked. Turkish President Recep Tayyip Erdoğan insists that he will proceed with an agreement to buy Russian S-400 anti-aircraft missile defense systems. President Trump’s administration says it won't deliver F-35s to Turkey if that arms deal goes through, as it would put Russia in a better position to track the stealth fighter jets. U.S. officials say Turkey could also face sanctions over the deal under a federal law designed to drive business away from the Russian defense industry.

“Turkey must choose: Does it want to remain a critical partner of the most successful military alliance in the history of the world?” Vice President Mike Pence said at a NATO conference Wednesday. “Or does it want to risk the security of that partnership by making reckless decisions that undermine that alliance?”

Pence’s stark statement came hours after Turkish Foreign Minister Mevlüt Çavuşoğlu declared at the same event that Turkey could do just that. “Turkey doesn’t have to choose between Russia or any others,” he said. “And we don’t see our relations with Russia as an alternative to our relations with others, and nobody, neither West nor Russia, should or can ask us to choose between.” Çavuşoğlu downplayed the value of President Trump’s offer to sell Turkey Patriot missile systems as an alternative to the Russian S-400s.

Linkevičius said Pence made a mistake in how he targeted Turkey, though he stressed, in diplomatic fashion, his “high respect” for the vice president’s address.

“It's always better to try to merge positions, so to say, from careful and patient dialogue, rather than bash publicly,” he said. “In general, I'm saying that this is not the way to talk to your ally.”

The Lithuanian envoy suggested that Western powers needlessly alienated Turkey through a lack of “empathy” in 2016 after a failed coup attempt, an error that U.S. officials have acknowledged while condemning Erdoğan for using the event as an excuse to crack down on journalists, academics, and other domestic critics. But he is concerned by Turkey's increasing partnerships with key Western adversaries.

“They are closely cooperating with Russia, sometimes Iran,” Linkevičius said. “It's not really encouraging.”

Szijjártó declined to “interfere” in the dispute but affirmed the entire alliance has a stake in the U.S.-Turkey relationship.

“If you are a mouse, don't stand there where the elephants are dancing,” the Hungarian diplomat said. “Our interest is that the two countries having the two biggest armies in NATO have a good cooperation.”

There will be “big damage” to the Western alliance if the S-400 sale goes through, Linkevičius said. “But let's not think about that yet,” he added. “We are not losing them yet.”