Secretary of State Rex Tillerson said Tuesday he is “keenly aware” of a security disagreement that is threatening to rupture the NATO alliance between the United States and Turkey, and said the U.S. is still standing by its ally.

“We are keenly aware of the legitimate security concerns of Turkey, our coalition partner and NATO ally,” Tillerson said at a counter-ISIS summit in Kuwait. “We will continue to be completely transparent with Turkey about our efforts in Syria to defeat ISIS, and we stand by our NATO ally in its counterterrorism efforts.”

The American effort to lead a coalition against the Islamic State in Syria has become a bone of contention with Turkey, because of U.S. cooperation with the Kurds, an ethnic minority who formed the most effective local fighting force in the region. Turkey worries that the Syrian Kurds will partner with Turkish Kurds who have fought a decades-long separatist war against Ankara. And so the Turkish military has launched a series of attacks on Kurdish positions in Syria, raising worries about a possible clash between U.S. and Turkish forces.

Tillerson’s Turkish counterpart warned of a fracture in the alliance. “Ties with the U.S. are at a very critical point,” Foreign Minister Mevlüt Çavuşoğlu told reporters in remarks published overnight. “We will either fix these relations or they will break completely.”

The disagreement over working with Kurds is one major piece of the tensions. Turkish President Recep Tayyip Erdogan’s government blamed a failed coup attempt in 2015 on the United States. Erdogan has demanded the extradition of alleged mastermind Fetullah Gulen, a Turkish cleric and former ally who lives in Pennsylvania, but former President Barack Obama and President Trump have refused. Erdogan cited the coup attempt to justify a series of authoritarian moves opposed by the United States. Then, dramatically, his security team beat protesters in Washington D.C. while he was traveling for a meeting with President Trump.

All the while, Russian President Vladimir Putin’s team has cultivated a closer relationship with the Turks. They both want the Syrian Kurds diminished, as Russia wants the United States and its partners to withdraw so that Syrian President Bashar Assad’s to regain total control of the country.

"To my mind, the Americans are trying to act through dangerous unilateral steps rather than by way of thoroughly developing general accord," Russian Foreign Minister Sergey Lavrov said Tuesday. "These steps increasingly look like part of the course towards cresting some quasi-state on a large part of Syrian territory — on the eastern bank of the Euphrates River up to the Iraqi border.”

Tillerson is going to try to allay such concerns in a trip to Turkey this week. “We believe there’s a way to work through, walk through, these problems, and that’s why the secretary is going to Ankara, to have those discussions; why a succession of senior U.S. officials have reached out, have engaged the Turks on the threat that they see, measures that might be taken to address that threat,” a senior State Department official told reporters on Monday.