Simmering tensions between Turkey and the United States spilled into the open on Wednesday as President Trump warned President Recep Tayyip Erdogan against the growing risk of conflict between the two nations. The Turkish president, for his part, demanded that the United States end its support for Kurdish militias.
The two men, both populists and unapologetic nationalists, spoke by telephone as Turkish forces attacked Kurdish militias in Syria. American officials said they were increasingly uneasy that the campaign against the Islamic State would be seriously undermined by the newest battlefront in a country that has been ravaged by war for nearly seven years.
Mr. Trump “urged Turkey to exercise caution and to avoid any actions that might risk conflict between Turkish and American forces,” the White House said in a description of the call. “He reiterated that both nations must focus all parties on the shared goal of achieving the lasting defeat of ISIS,” or the Islamic State.
His tough tone with Mr. Erdogan was an abrupt reversal from a White House briefing just a day earlier, where senior administration officials suggested that the United States would side with Turkey, a NATO ally, in disputes with Kurdish forces that have fought the Islamic State with direct support from Washington.
Turkey has fought for decades against Kurdish insurgent groups that Ankara considers terrorist threats. Kurds in the region — most notably in Turkey, Iraq and Syria — dream of creating an independent state even as they seek greater political and cultural rights.
In a speech on Wednesday, Mr. Erdogan said there was no difference between the Islamic State and Kurdish militias, and questioned “the humanity of those who accuse Turkey of being an invader and support an organization that has the blood of tens of thousands of innocent children, women, elderly people and innocents on its hands.”
The shift in tone at the White House grew out of what officials described on Wednesday as frustration over months of failed efforts to mollify Mr. Erdogan, including changes in military strategy and reassurances about Kurdish intentions on the battlefield. And it marked the end of a year of wooing of Mr. Erdogan, whom Mr. Trump has repeatedly praised despite the Turkish leader’s authoritarian crackdown at home.
In Syria, a Turkish warplane launched a shell that fell near the northern city of Manbij, where United States troops have been training and equipping Kurdish forces that are holding the strategic city. American officials are increasingly worried that a wide-scale attack on Manbij would bring troops from the United States and Turkey into direct conflict.
“The shell fell in an empty place. It didn’t cause any damage,” said Sherfan Darwish, a spokesman for the Manbij Military Council. “In general, the fronts are quiet.”
The main brunt of an ongoing Turkish attack targeted the Syrian border town of Afrin, which is controlled by Kurdish forces. The United Nations said the recent fighting had displaced at least 5,000 people in and around the Afrin district. Of the area’s 323,000 residents, only some of whom are Kurds, more than a third have been displaced from elsewhere in Syria.
In one video posted online, an Afrin resident stood beside his destroyed house and cursed the Turkish president.
The Turkish attacks led Kurds in Syria and elsewhere to worry that the United States would again abandon them. They harked back to massacres of Kurds in Iraq by the former dictator Saddam Hussein — including a 1988 chemical weapons attack in Halabja that killed 5,000 — that they say Washington either failed to prevent or was complicit in covering up.
Nisrine Abdallah, the spokeswoman for the Kurdish Women’s Union, described the situation in Afrin as “miserable” and said the Turkish bombing campaign against civilians there was intensifying. She blamed the Americans.
“For five days Afrin is being brutally attacked. We didn’t see any movement by the Americans,” Ms. Abdallah said, adding: “I wish the U.S. would take a clear position, at least on the humanitarian level.”
Even as the White House has sought to strike a balance between Turkey and Kurds in Syria, the Pentagon has maintained its support for the Kurdish militias it has fought with in eastern Syria.
Without logistical support from Kurdish forces, the nearly 2,000 American troops in Syria would be unable to continue in almost any effective role, according to senior military officials. Kurdish militias are expected to play a pivotal role in coming months in defeating the last pockets of several hundred Islamic State fighters in the country.
During Wednesday’s phone call, the White House said, Mr. Trump also voiced concern about “destructive and false rhetoric coming from Turkey.” Additionally, he noted prolonged detentions of American citizens and local employees in Turkey, a rare reference by the White House to human rights concerns.
The United States turned to Kurdish forces when the Islamic State shocked the world in June 2014 after it seized control of Mosul, Iraq’s second-largest city, and then rolled across much of Syria. With Iraq’s army shattered and Syrian troops battling political opponents, Kurdish militias became the most dependable ground force available against the extremists.
Most of the weapons provided to Kurdish militias are Soviet in design and would blend with most other firearms in the region, making them extremely difficult to track and spot even during peacetime — never mind during the chaotic environment of a war.
Strains between Turkey and the United States have been growing for years. They accelerated after a failed 2016 coup attempt in Turkey led Mr. Erdogan to tighten his grip on the nation and ramp up rhetorical attacks on the United States and Europe. The Turkish government said the coup was led by Fethullah Gulen, an Islamic cleric who is living in exile in the United States.
Both countries view the fight against the Islamic State as important. But tensions have begun to resurface as the battle against the Islamic State appears to be near an end.