SANLIURFA, Turkey — Fighting lit up the sky early Thursday as Turkish troops pressed their air and ground offensive against United States-allied Kurdish fighters in northern Syria. At least 16 Kurds were reported to have been killed, one monitoring group said.
Members of the Kurdish-led Syrian Democratic Forces were killed in the Tel Abyad and Ras al-Ain areas of northeastern Syria, along with six attackers of unknown identity, according to the Syrian Observatory for Human Rights, a conflict monitor based in Britain. American troops had withdrawn from both areas on Monday.
An additional 33 members of the Syrian Democratic Forces were wounded, the monitoring group said.
The Turkish military’s move into Syria began on Wednesday, following President Trump’s decision on Sunday to pull American troops out of Turkey’s way, despite disagreement from his own military officers and State Department.
Mr. Trump condemned Turkey’s operation as a “bad idea” on Wednesday and said this week that Turkey, a NATO ally, would face economic punishment if it did anything he considered “off limits.”
He has not said what such actions might be, but some analysts have considered the killing of Kurdish fighters a potential red line, both for Mr. Trump and for members of Congress who opposed the pullback of American troops. The troops’ presence had been considered a deterrent to an invasion by Turkey, which has long sought to root out American-allied Kurdish forces who were instrumental in the American fight against ISIS.
In the northern Syrian town of Tel Abyad, one of the spots where the fighting was heaviest, videos by residents recorded the rattle of gunfire and streaks of tracer fire over the town.
The Anadolu news agency reported that cannon fire into Tel Abyad decreased through the night. Land forces crossed into the city, and by Thursday morning the streets were silent.
In a statement on Wednesday, Turkey’s defense minister, Hulusi Akar, said the aims of the operation were to “ensure the security of our borders and the safety of our people,” naming Kurdish militias and Islamic State militants as threats.
Six hours of airstrikes ensued, followed by Turkish ground troops crossing the border into Syria.
The fighting threatens to create a humanitarian crisis for hundreds of thousands of people who have been cut off from Syrian assistance for years. Most rely on the Kurdish forces and aid groups for basic services. Civilians jammed roads while fleeing with their possessions on Wednesday.
Henrietta Fore, the executive director of Unicef, said in a statement on Wednesday that the military escalation would have “dramatic consequences” on the ability to provide aid.
“I urge all parties to protect children and the civilian infrastructure on which they depend, in accordance with international human rights and humanitarian law,” she said. “The use of explosive weapons in populated areas causes unacceptable harm to children.”