The Islamic State appeared to suffer an important setback on Monday when American-backed militia fighters in Syria seized the main route that connects Raqqa, the Islamic State’s de facto capital, to its territory in southeastern Deir al-Zour Province.
The development essentially severed the last remaining access for supply deliveries to Raqqa and may have eliminated an escape route for Islamic State fighters.
Syrian government forces lost control of Raqqa in 2013 to the opposition, and the Islamic State captured the city later that year. Raqqa was the Islamic State’s most important territorial triumph at the time, and the extremist movement regards the city as the center of its self-proclaimed caliphate.
The seizure of the Raqqa exit route on Monday by the American-backed militia, known as the Syrian Democratic Forces, was confirmed by a Pentagon spokesman, Capt. Jeff Davis, and by the Syrian Observatory for Human Rights, a monitoring group. The Syrian Observatory said the militia fighters were just five miles outside the Raqqa city limits.
An assault on Raqqa to expel the Islamic State, also known as ISIS or ISIL, is widely expected in coming weeks, and if successful, it would deliver the organization’s biggest defeat in its short and violent history.
The impending assault also represents a test of the complex array of competing forces in Syria fighting the Islamic State — forces that also could turn on one another.
They include rebel Kurdish and Arab militia members trained and equipped by the United States, Turkish soldiers, the Syrian forces of President Bashar al-Assad and his militia allies supported by Russia and Iran.
Mr. Assad’s forces, which recaptured the city of Aleppo from insurgents in December after a prolonged siege, have been rapidly advancing east toward Raqqa in the past few weeks, Reuters reported Monday.
In a sign that the Islamic State is preparing for a possible retreat, male civilians in Raqqa were recently ordered to start dressing similarly to the group’s fighters, according to the Syrian Observatory and a second monitoring group known as Raqqa Is Being Slaughtered Silently.
By blending into the population, Islamic State fighters not only improve their escape prospects, but make distinguishing civilians from combatants difficult for the American-backed coalition of aerial forces hitting targets around Raqqa.
Activists from Raqqa Is Being Slaughtered Silently reported last month that coalition warplanes had destroyed Euphrates River bridge connections into Raqqa. So the Syrian Democratic Forces’ seizure of the land route from Raqqa to Deir al-Zour was seen as a major step in isolating the city.
President Trump has said his objective in Syria is to eradicate the Islamic State, but precisely how he intends to accomplish that goal has not been made clear.
The risk of clashes between the Syrian Democratic Forces and Turkish soldiers in Syria has increasingly worried American military officials. Turkey, a NATO ally of the United States, regards the Kurdish component of the militias as an enemy aligned with Turkey’s Kurdish separatists.
Over the weekend, American military officials said the United States had strengthened its contingent of Syria-based forces to help deter clashes around Manbij, a town in northern Syria near the Turkish border, which Kurdish militia members helped capture from the Islamic State in August.
Turkey has complained that the Kurds have not vacated Manbij as promised.
Fighting in the Syrian civil war, which is about to enter its seventh year, has declined in recent weeks, partly because of a tenuous cease-fire negotiated by Russia and Turkey when Aleppo was retaken by Mr. Assad’s forces.
But the prospects for a political settlement remain remote, and the humanitarian crisis has only worsened. Hundreds of thousands have been killed since the conflict began in March 2011 as an uprising against Mr. Assad. Roughly five million people have fled the country, and millions more have been displaced.
In a new sign of the toll, Save the Children said in a report issued Monday that the war traumas suffered by Syrian children had increased their long-term risks of suicide, heart disease, diabetes, substance abuse and depression. The report, based on interviews with more than 450 children, adolescents and adults across the country, described a growing child-health crisis that could leave many suffering a condition known as toxic stress.
“The children we spoke with in Syria are terrified to play outside, afraid to go to school, and soiling themselves when they hear a loud noise,” Carolyn Miles, the president and chief executive of Save the Children, said in releasing the report.