The fuse may be lit on a major new crisis in war-torn Syria, as Turkish forces and their local allies push deeper into Syrian territory and threaten Manbij, a city where U.S. troops are deployed.

Reuters noted on Monday that Turkish officials, up to and including President Recep Tayyip Erdogan, have warned foreign forces to clear a path if and when Turkey makes a move on Manbij.

“This is what we have to say to all our allies: don’t get in between us and terrorist organizations, or we will not be responsible for the unwanted consequences,” Erdogan said ominously in January.

However, the United States has given no indication it is prepared to withdraw from the town. U.S. Secretary of State Rex Tillerson is reportedly bracing himself for a “difficult conversation” when he arrives in Ankara this week.

“We will discuss these issues during Tillerson’s visit, and our ties are at a very critical stage. Either we will improve our ties, or they will completely deteriorate,” Turkish Foreign Minister Mevlut Cavusoglu said on Monday, offering a preview of just how difficult that conversation is likely to be.

“Missing trust needs to be restored. The reason for that missing trust is the U.S.,” Cavusoglu charged.

“Our demands from the U.S. are clear and have already been conveyed. We no longer want to hear about promises, we want to hear about concrete steps. Trust needs to be rebuilt so we can start to talk about some issues. U.S. actions are the reason behind the loss of trust,” he said.

Turkish officials have criticized the United States for arming the Kurdish YPG militia in Syria in the war against the Islamic State and refusing to reclaim those weapons, for refusing to pull out of Manbij, and for failing to persuade the YPG to back away from Manbij and the currently besieged town of Afrin. Turkey views nearly all Kurdish militia forces as allies of the violent PKK separatist movement in Turkey.

A senior foreign policy adviser to President Erdogan denounced last week’s visit to Manbij by U.S. military commanders as a “gung-ho gesture” and a “flippant and provocative display.”

The display was intended to be provocative. Coalition commander Lt. Gen. Paul Funk and special ops commander Maj. Gen. Jamie Jarrard were flying huge American flags on their vehicles when they rolled into Manbij last Wednesday to make sure the Turks and their allies knew they were there.

“You hit us, we will respond aggressively. We will defend ourselves,” promised Funk, who made a point of standing on a roof surrounded by Arab and Kurdish fighters so Turkish troops and their Syrian allies could get a good look at him.

The forces controlled by the Manbij Military Council are mostly Arab, in fact, so Turkey would likely touch off a regional powder keg by attacking the city even if the Americans withdrew. U.S. officials attempted to host meetings between the Turkish military and the Manbij Military Council to show them it’s not an all-Kurdish operation, but according to Funk, the Turks turned down an invitation to such a meeting earlier in the year.

The Manbij Military Council stated last week that if Turkey attacks the city, it will be “responded to with the same historic resistance as Afrin,” which has not been conquered as swiftly as the Turks anticipated.

“This aggression that practices ethnic cleansing and mass genocide against our people in Afrin is aimed for producing terrorist groups in areas where there is peace and coexistence,” the MMC added.

Manbij Military Council leader Ibrahim Hamdan praised Syrian youth groups for helping to oppose the Turkish invasion on Monday and said “we are all in one trench” when it comes to defending hard-won territory against acts of aggression.

“We, as members of Manbij Military Council, have our hearts with Afrin and we are confident that young people are the effective force to deal with Erdogan,” Hamdan said.

“Erdogan uses mercenaries after his failure to remove the revolution from within us. We are now facing a new organized terrorist army,” he said, calling the Turkish president out by name.

The crisis point might well have been reached already if Turkey’s offensive had not gotten bogged down in Afrin. As Hamdan’s address indicates, resistance in Afrin is becoming a rallying cry for other Kurdish forces and their allies, foreshadowing a tough fight if Turkey goes any deeper into Syria.