Turkish President Recep Tayyip Erdoğan’s determination to purchase Russian anti-aircraft missile systems could cripple his country’s participation in the NATO alliance, Vice President Mike Pence warned Wednesday.

“Turkey must choose: Does it want to remain a critical partner of the most successful military alliance in the history of the world?” Pence said at NATO Engages, an event hosted on the sidelines of the NATO summit in Washington. “Or, does it want to risk the security of that partnership by making reckless decisions that undermine that alliance?”

Pence’s rebuke raised the stakes of Turkey’s insistence over buying Russian S-400s to deploy alongside NATO’s most advanced stealth fighter jet. Pence reiterated that the United States would not deliver the F-35 warplanes to Turkey if Erdoğan sticks with the S-400s, just hours after the top Turkish diplomat demanded the planes while downplaying the value of Trump’s offer to sell alternative U.S.-made air defense systems.

“Turkey is part of this [program] and we have [been] introducing many parts of this F-35,” Turkish Foreign Minister Mevlüt Çavuşoğlu said at the same forum. “So this is not like U.S. is producing something and U.S. is selling to NATO allies, including Turkey. So, we are part of this program, so it shouldn’t be that simple.”

Çavuşoğlu also denied Turkey is creating a rift within NATO. “Turkey doesn’t have to choose between Russia or any others,” Çavuşoğlu 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 [them].”

European countries have a heightened sense of the danger posed by Russia because of the violence in Ukraine since 2014, when the Kremlin annexed Crimea and sent unmarked special forces into the eastern part of the country. Çavuşoğlu suggested the conflict arose, in part, from Western demands that Ukraine spurn Russia.

“And, it happened with Ukraine, and look at what happened,” he said at the event, which is being hosted by the Atlantic Council, the German Marshall Fund, and the Munich Security Conference. “The West asked Ukraine to choose only them and Russia did the same thing, vice versa, so Turkey cannot be put in the same situation and we need to balance our relations with everybody.”

Çavuşoğlu’s comments drew an immediate rebuke from a Ukrainian official at the event, which was structured as a question-and-answer session.

“You said that Russia is pulling my country to one side, NATO is pulling it to another side,” Ivanna Klympush-Tsintsadze, Ukraine’s vice prime minister for European and Euro-Atlantic Integration, told the Turkish official. “It’s our decision. We have decided to move to NATO.”

Çavuşoğlu expressed optimism President Trump would authorize the delivery of the jets, saying he had heard “contradicting statements” from different parts of the U.S. government. Pence replied with an unambiguous endorsement of the Pentagon’s decision this week to halt delivery of any equipment related to the F-35.

“The Pentagon made clear earlier this week and I repeat today: If Turkey completes its purchase of the Russian S-400 missile system, Turkey risks expulsion from the joint F-35 program” Pence said.