US Vice President Joe Biden tried to patch up ties with President Recep Tayyip Erdogan's government this week, but the conflict in Syria has forced Washington into a delicate balancing act.
On the ground, the US strategy relies on using the Kurdish YPG militia backed by American special forces advisors and coalition air power to take the fight to the IS group.
NATO member Turkey is a nominal part of the anti-IS coalition, but regards the YPG as part of the same "terrorist" movement as the PKK Kurdish separatist group waging a guerrilla war within its borders.
This week, Ankara sent troops and allied Syrian rebel fighters to seize the Syrian border town of Jarabulus from IS group fighters.
But Erdogan also aimed to deny it to the Kurds, who have advanced across the Euphrates River into the region as the dominant faction in the US-backed Syrian Democratic Forces (SDF).
Now Turkey is demanding the Kurds retreat across the river, announcing on Thursday that its military had begun shelling YPG positions north of the town of Manbij, which the Kurds seized this month.
The US dilemma was underlined in a pair of tweets from US diplomat Brett McGurk, President Barack Obama's special envoy coordinating the coalition fighting the Islamic State group.
"We support our NATO ally Turkey in protecting its border from ISIL terrorists, and struck ISIL targets near Jarabulus earlier today," he wrote, using the US government's acronym for the IS group.
"We also support the Syrian Democratic Forces which have proven a reliable and extremely capable force in the fight against ISIL," he added, reflecting Washington's reliance on Kurdish manpower.
- Tacit approval -
US diplomats have successfully maintained that dual loyalty -- despite protests from Ankara -- for months as the coalition has begun to recapture ground from IS group forces in Iraq and Syria.
But Ankara will only tolerate open US support for the Kurds for so long, and analysts see this week's Jarabulus offensive as a clear warning to both Washington and the YPG that it is prepared to act.
Erdogan, they warn, would not have launched the operation without the tacit approval of Russia, Syrian strongman Bashar al-Assad and his ally Iran -- showing Turkey has contacts outside the coalition.
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"I don't think Turkey is ready to let go of the United States and its NATO partnership just yet," said Merve Tahiroglu, a researcher on Turkey at the Foundation for the Defense of Democracies (FDD).
"But what yesterday's operation was really about was to show the Kurds and the United States that Turkey has options," she added.
"They are carrying out this operation... so that the Kurds don't have to, so that the United States doesn't have to do it, and that the Americans don't do it with the Kurds."
The increased Turkish assertiveness comes amid a wave of anti-Americanism, stoked by Erdogan's loyal supporters in the media, in the wake of a failed coup d'etat attempt last month.
America has denied Turkish reports it was somehow behind the putsch, but Erdogan has angrily demanded it extradite US-based cleric Fethullah Gulen, the plot's alleged mastermind.
While officials greeted Biden with smiles and handshakes, pro-government media have stirred up Erdogan's supporters with tales of Western skullduggery and his popularity has grown.
For former Turkish member of parliament Aykan Erdemir, now an FDD fellow, the orchestrated anger reflects a move away from Turkey's formerly pro-Western orientation.
"Turkey is moving into what I would call a more Middle Eastern modus operandi," he said.
"That is, a lot of incitement in government-funded media, strong anti-Americanism, strong anti-Westernism... coupled with quite a warm reception in one-on-one settings.
- Public anger -
Erdogan does not really fear the United States tried to overthrow him, but is riding a wave of public anger and exploiting "Turkey's deep-rooted xenophobia, anti-Semitism and anti-Western sentiment," Erdemir said.
Three-quarters of Turks have a negative view of the United States, and Erdogan appears to believe he can control and exploit the angry mood, according to a Pew poll conducted in October 2014.
That's a mistake, Erdemir argues.
"Once you fuel these fires, you never know how and where it will end," he said. "And that's why I'm concerned about Turkey's trans-Atlantic orientation, I'm concerned about Turkey-EU relations."