BEIRUT — The Turkish invasion of northeastern Syria has forced the U.S. military and its Syrian Kurdish allies to significantly curtail their shared military operations against the Islamic State at a critical moment in the ongoing fight to stamp out the group’s residual presence, ­creating an opening for the militants’ comeback, U.S. and Kurdish officials say.

Hundreds of fighters with the Kurdish-led Syrian Democratic Forces (SDF) have been relocated to the front lines with Turkey and away from areas where the anti-Islamic State operations were focused, drawing manpower and resources away from the daily raids and missions that have thwarted an Islamic State revival. 

Defense Secretary Mark T. Esper said on Friday that the United States had not abandoned its Kurdish allies and that the 1,000 troops deployed there would continue hunting down the remnants of the Islamic State. The militant group lost territorial control of its self-proclaimed caliphate earlier this year but is making strenuous efforts to resuscitate its organizational structures across Syria and Iraq.

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“We will continue to work with the 80 members of the Defeat-ISIS coalition and the Syrian Democratic Forces to ensure the defeat of ISIS,” he said, using the acronym for the Islamic State.

U.S. officials privately acknowledged, however, that the tempo of operations by the Kurds, Washington’s main partner in Syria, against the Islamic State has “significantly tapered off” since the Turkish offensive began, according to one official who spoke on the condition of anonymity because he was not authorized to speak to the news media.

A senior SDF official said the anti-Islamic State fight had come to a complete halt, because U.S. troops cannot operate without their SDF partners on the ground, and the SDF is unable to participate in missions while also confronting Turkey. “We are focusing on the Turkish fight,” said the official, speaking on the condition of anonymity per ground rules established by the SDF.

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The most immediate concern is that Islamic State fighters and their families will escape from any of the 20 or so prisons and camps dotted around SDF-held territory. Only about 1,500 of the 10,000 fighters, including foreigners, Iraqis and Syrians, are detained in prisons in the border area. The SDF continues to guard all the prisons and camps where Islamic State fighters or family members are being held, it said. 

In the first instance of a prisoner escape since the invasion began, five Islamic State detainees managed to flee Friday during the panic triggered by a mortar strike on a prison on the outskirts of the Syrian city of Qamishli, according to SDF officials and CCTV video. Tensions have also been rising at the al-Hol camp near the Iraqi border, which houses 70,000 women and children displaced by the fighting. 

There are also thousands of Islamic State fighters still on the run. They have managed to blend in among the Arab communities they once ruled or are camped out in desert hideouts, officials say. 

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The most worrisome pockets are hundreds of miles to the south of the front lines with Turkey, in the province of Deir al-Zour and along the Euphrates River valley leading to the Islamic State’s former capital of Raqqa — overwhelmingly Arab areas that were already chafing under the rule of the Kurdish-dominated SDF.

Until this week, U.S. troops and their SDF partners had been conducting raids and missions on a regular basis to root out sleeper cells, financiers and recruiters, and they believed they were managing to stay a step ahead of the militants’ efforts to regroup, according to U.S. officials. In the week immediately preceding the Turkish incursion, SDF forces alongside coalition partners captured three Islamic State figures linked to explosive attacks and militant sleeper cells, the U.S. military said.

The progress could be reversed with the pause in the effort, risking a vacuum that the Islamic State could exploit to step up its attacks or perhaps try to seize territory again, said the SDF commander, Gen. Mazloum Kobane, who uses a nom de guerre and is known simply as Mazloum. “We have to keep them under pressure so that they cannot regroup,” he said. 

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There had been no reason until this week to believe the Islamic State was in any position to take over territory, as it did in 2014, when its sweep through Iraq and Syria brought U.S. troops back to the region, he said. Although scattered attacks still take place, they are small in scale. A bomb outside a restaurant in the town of Qamishli that killed three people was claimed by the Islamic State on Friday. An attack in Raqqa overnight Monday was initially presented by the SDF as a major assault, but it turned out to involve a lone suicide bomber who killed himself and one SDF fighter at a security post.

The group either lacks the capacity to launch the kind of sustained, repeat suicide bombings that preceded the rise of its “caliphate” in 2014, whether in Iraq or Syria, or is biding its time until it feels the moment is right, U.S. officials say. 

Instead, the group has been focused on reviving the financial and recruitment networks that helped swell its ranks and fueled the insurgency in the past, according to U.S. and Kurdish officials. 

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Although the militants lost control of their territory, their organizational structures remain largely intact, said Hassan Hassan of the Center for Global Policy, who is from Deir al-Zour. 

“Vigilance is important. ISIS is coming back slowly, but the danger is real,” he said. “Their organization still functions. You would imagine it shattered, but it seems to be robust. It’s not back yet, but they are rebuilding and still have that kind of fear and ability to scare and terrify people in the areas.”

In recent weeks, the militants have shifted their focus from carrying out small-scale attacks on security forces to intimidating residents and abducting or killing those who collaborate with the SDF in Arab areas, said Mazloum. Doubts about the durability of U.S. support for the Kurds could further deter local residents from cooperating with U.S. and SDF efforts to stabilize communities by restoring local governance and services.

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The repercussions also could be felt in neighboring Iraq, which has been hit in the past 10 days by a wave of popular unrest that risks detracting attention from the Islamic State fight. There, the Islamic State has had more time to regroup from its defeats and has made greater progress in reactivating its networks, military officials say. Militants who are thought to have escaped from the final defeat at the battle of Baghouz in Syria have been arriving in Iraq to reinforce local cells, including foreigners, said an Iraqi Kurdish intelligence official, speaking on the condition of anonymity because he is not authorized to speak to the press.

The Iraqi army has sent reinforcements to the Syrian border to guard against a spillover of chaos that could bring a surge of Islamic State fighters into Iraq, an Iraqi army spokesman said. But already it is clear that Islamic State operatives are reestablishing a degree of freedom of movement across an arc of territory stretching from eastern Syria to eastern Iraq, said the intelligence official.

Pentagon officials said they had not detected that the protests engulfing Baghdad and much of southern Iraq had affected operations against the Islamic State. But the Kurdish intelligence official said it has already become clear that the upheaval was distracting government attention from efforts to coordinate much-needed operations against the expanding Islamic State presence.

Souad Mekhennet in Washington contributed to this report.

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