AKCAKALE, Turkey — Turkish forces and their Syrian Arab allies swept into northern Syria on Thursday, killing at least 23 Kurdish fighters in a sharp escalation of a campaign against a Kurdish-led militia that had fought alongside the United States against the Islamic State.
The Turkish assault, which began on Wednesday after President Trump acceded to it and moved American troops out of the way, has pulled the militia out of its fight against the remnants of the Islamic State in Syria.
Three American officials said Thursday that all counterterrorism operations by the militia, the Syrian Democratic Forces, had been suspended.
The officials, speaking on condition of anonymity to discuss confidential military assessments, said the militia had been carrying out several dozen missions per day, some in conjunction with American Special Forces.
Opponents of the Turkish invasion in Washington and Europe have expressed concern that the operation would weaken efforts to keep the Islamic State fighters from regrouping.
The Turkish forces also seized at least 11 villages clustered around two Kurdish-held border towns that stand about 75 miles apart, according to Anadolu Agency, a Turkish state-run news agency.
At least 11 civilians in Kurdish-held areas were killed and 28 wounded in the Turkish attacks on Thursday, according to the Rojava Information Center, an activist group in northeastern Syria.
The Syrian Democratic Forces appeared to be striking back.
Shells and rockets landed in several Turkish border towns, killing four civilians, including an infant, and wounding 70.
At least one Turkish soldier was killed, according to a tweet by a lawmaker from Turkey’s governing party.
More than 60,000 Syrians in Kurdish-held territory have fled their homes since Wednesday, according to the Syrian Observatory for Human Rights, a conflict monitor based in Britain, and United Nations officials.
Turkey’s foreign minister, Mevlut Cavusoglu, said that Turkish forces did not plan to go farther than about 19 miles into Syrian territory, a distance he said was needed to stop Kurdish fighters from firing missiles into Turkey.
Mr. Cavusoglu said that ultimately Turkey planned to seize a corridor stretching for hundreds of miles along the Turkish-Syrian border but he did not say when. “We will do that in time,” he said in an interview on Turkish television.
Turkish military analysts said this week that the initial plan was to seize only the 75-mile stretch between the towns of Tel Abyad and Ras al Ain.
At least one Syrian town less than a mile from the border, Al Yabseh, fell with no resistance to a brigade of around 1,000 Turkish-backed Syrian Arab fighters.
“We captured the town an hour ago without any clashes,” Al-Hareth Dahdouni, 31, a member of the militia said by telephone from inside Syria. “The town was totally empty. It is just one minute away from the border.”
Turkish troops appeared to have stayed within a few miles of the Syrian border, according to information provided by the state-run Anadolu Agency.
On the Turkish side of the border, the military buildup continued steadily in full view of residents.
A long convoy of military trucks carrying armored personnel carriers under tarpaulins entered the Turkish border town of Akcakale after dark. Soldiers in camouflage stood among dozens of armored vehicles parked in a base beside the main road.
The Turkish Army used cranes to remove parts of a concrete border wall, allowing Turkish troops, Turkish-backed Syrian Arab rebels and military vehicles to enter northern Syria.
In Akcakale, three sharp explosions filled streets around the town’s police headquarters with smoke, sending pedestrians fleeing for cover and armored police vehicles barreling through the streets.
The Turkish military responded with salvo after salvo of artillery fire throughout the afternoon, pounding the Syrian town of Tel Abyad visible across the border where the Kurdish-led militia appeared to be still ensconced.
Tel Abyad is one of two small Syrian border towns that the Turkish military has made its first target, pounding it with artillery and airstrikes on Wednesday.
By late afternoon black smoke rose from a line of burning buildings along the ridgeline of the town, ballooning in the sky and blocking out the horizon.
Just to the west, a building in Syria that Turkish residents said was an arms depot, was also in flames. Choking black smoke drifted back into Turkey over the village of Upper Arican.
The police were urging residents to evacuate.
“They said it was safer further inside Turkey, but we don’t want to leave our house,” Ayse Bayrak said. “No one wants to leave their house.”
But many did, on both sides of the border, cramming into cars and pickup trucks and searching for safety.
The fighting marks a new stage in the eight-year-old Syrian civil war that began as a wave of protests against President Bashar al-Assad of Syria, but which has since escalated into a jumble of overlapping conflicts involving foreign armies and an array of local militias, including former Syrian government officers, Islamist extremists and Kurdish nationalists.
The United States joined forces with a Kurdish-led militia, the Syrian Democratic Forces, to clear northeastern Syria of militants from the Islamic State. Kurdish groups seized the opportunity to carve out an autonomous statelet in northeastern Syria.
The Kurdish presence, abutting the Turkish border, enraged the Turkish government, which considers the Kurdish-led militia an enemy because of its ties to a Kurdish guerrilla force inside Turkey.
For several years, a small American force kept the peace between the Syrian Kurds and Turkey — until President Trump’s sudden decision on Sunday to pull American troops out of Turkey’s way, despite qualms from his own military officers and State Department officials, and criticism from Republican politicians.
On Thursday afternoon, President Recep Tayyip Erdogan of Turkey said the invasion was an attempt to preserve Syria’s long-term sovereignty and clear the area of not just Kurdish fighters but also of the remnants of the Islamic State.
‘‘The aim of Operation Peace Spring is to contribute to the territorial integrity and political union of Syria,” Mr. Erdogan said in a speech to his political party, using the Turkish military’s name for the invasion.
He dismissed concerns that the mayhem of the invasion would risk allowing thousands of Islamic State militants detained by the Kurdish-led militia to escape.
“We will keep in jail the ones who should be kept in jail and send the other ones to their countries of origin, if those countries accept them,” he said.
The American officials said that the Syrian Democratic Forces were still in control of the prisons, though they had scaled back the number of guards.
President Emmanuel Macron of France condemned the offensive on Thursday, calling for Turkey to end it “as quickly as possible.”
A group of European countries, including Germany, France and Britain, issued a statement calling for Turkey to “cease the unilateral military action,” saying it would not address Turkey’s security concerns and would “undermine the stability of the whole region, exacerbate civilian suffering” and increase the number of displaced people and refugees.
Both Mr. Macron and the European statement warned that the Turkish action could aid the resurgence of the Islamic State.
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.
The impact of the assault was “a lot worse, a lot more dramatic” than aid organizations had feared, Panos Moumtzis, the United Nations humanitarian aid coordinator in Syria, said in a telephone interview. “The protection of civilians is now the biggest concern.”
Some aid organizations had already evacuated personnel from the area, he added.
Hundreds of thousands of civilians “are now in harm’s way,” Filippo Grandi, head of the United Nations refugee agency, said in a statement.
Henrietta Fore, the executive director of UNICEF, said 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.”
Inside Turkey, families also began loading up their belongings and leaving town to travel away from the border. “We are going west because people were hit on the east side of town,” said Ayse Kaya as she piled her family into a small car.
But others, including Syrian refugees now living in Turkey, were less concerned.
“I have seen this before, I am from Syria, I am going to prepare a shisha,” said Mustafa Ali, an elderly man in a long cotton robe, referring to a traditional water pipe. “I came from Aleppo, I saw many of these in Aleppo. I am going to prepare a pipe at home.”
Carlotta Gall reported from Akcakale, and Patrick Kingsley from Istanbul. Reporting was contributed by Eric Schmitt from Washington; Daniel Victor from Hong Kong; Karam Shoumali from Berlin; Hwaida Saad from Beirut, Lebanon; Aurelien Breeden from Paris; and Nick Cumming-Bruce from Geneva.