Turkish troops shelled US special forces who are still on the ground during the conflict in northern Syria, American officials say.
The incident happened near the city of Kobani on Friday night, just days after President Donald Trump said he would pull US soldiers out of the region.
The Pentagon claims Turkey was aware that US forces were in the area.
US officials are reportedly split over whether the shelling was a mistake or a deliberate attempt to pressure the Americans into leaving the war zone where Turkey is attacking the Kurds.
A senior Pentagon official told Newsweek that the US troops considered firing back in self-defence because the Turkish forces’ shelling was so heavy.
The artillery shells hit several hundred metres from the American soldiers and none of the US special ops members were injured, the Pentagon said.
The American troops withdrew once the shelling had stopped.
Navy Captain Brook DeWalt, a Pentagon spokesman, said: “US troops in the vicinity of Kobani came under artillery fire from Turkish positions at approximately 9pm.
“The explosion occurred within a few hundred metres of a location outside the Security Mechanism zone and in an area known by the Turks to have US forces present.”
Turkey’s Defence Ministry denied that its soldiers had opened fire on US or coalition troops in Syria and said its troops were targeting Kurdish fighters who were nearby.
It said it “ceased fire upon receiving information from the US”.
CNN said the US officials it spoke to are divided over whether the shelling was a deliberate attempt to force American forces to leave the area or if it was a mistake by Turkey’s military.
Kobani is about 35 miles west of the main area of conflict.
Meanwhile, Turkish forces stepped up their bombardment around a town in north-east Syria on Saturday, the fourth day of an offensive against a Kurdish militia.
The United States has ramped up its efforts to persuade Ankara to halt the incursion against the American-backed Kurdish forces, saying Ankara was causing “great harm” to ties and could face sanctions.
Turkey opened its offensive after Trump spoke by phone on Sunday with Turkey’s President Tayyip Erdogan and withdrew US troops who had been fighting alongside Kurdish forces.
On Friday evening, Erdogan dismissed mounting international criticism of the operation and said Turkey “will not stop it, no matter what anyone says”.
On the frontlines, thick plumes of smoke rose around Syria’s Ras al Ain, one of two border towns targeted in the offensive, as Turkish artillery targeted the area on Saturday, said a Reuters reporter across the frontier in the Turkish town of Ceylanpinar.
Intense gunfire also resounded from within the town of Ras al Ain itself, while warplanes could be heard flying overhead, he said.
It was quieter at Tel Abyad, the operation’s other main target some 75 miles to the west, with only occasional shell fire heard in the area, another Reuters reporter said.
A war monitor gave a death toll of more than 100 from the first days of the assault. The United Nations said 100,000 people had fled their homes.
On Saturday morning, Turkey’s Defence Ministry said that 415 YPG militants had been “neutralised” since the operation began, a term that commonly means killed.
The Syrian Democratic Forces (SDF), with the Kurdish YPG as its main fighting element, now holds most of the territory that once made up Islamic State’s “caliphate” in Syria, and has been keeping thousands of fighters from the jihadist group in jail and tens of thousands of their family members in camps.
The Kurdish militia say the Turkish assault could allow the jihadist group to re-emerge as some of its followers were escaping from prisons.
In its first big attack since the assault began on Tuesday, Islamic State claimed responsibility for a deadly car bomb in Qamishli, the largest city in the Kurdish-held area, even as the city came under Turkish shelling.
Five Islamic State fighters fled a jail there, and foreign women from the group being held in a camp torched tents and attacked guards with sticks and stones, the SDF said.