Children stand outside the small house their families now share in southern Turkey after fleeing a village in the Hasaka countryside in Syria. (Alice Martins/for The Washington Post)

The unexpected rout of Islamic State forces across a wide arc of territory in their northeastern Syria heartland has exposed vulnerabilities in the ranks of the militants — and also the limits of the U.S.-led strategy devised to confront them.

Islamic State fighters have been driven out of a third of their flagship province of Raqqa in recent weeks by a Kurdish-led force that has emerged as one of the most effective American partners in the war. The offensive, backed by U.S. airstrikes, has deprived the militants of control of their most important border crossing with Turkey and forced them onto the defensive in their self-proclaimed capital of Raqqa city, something that would have been unthinkable as recently as a month ago.

The advance has shifted the focus of the fight from Iraq to Syria for the first time in months. A blitz of 18 coalition airstrikes against Raqqa over the weekend took out bridges and roads used by the Islamic State to move supplies to battlefronts elsewhere. The air attack was one of the most intense in Syria, according to a Pentagon statement and activists in Raqqa.

On Monday, President Obama cited the recent gains in Syria as evidence of progress. “When we have an effective partner on the ground, ISIL can be pushed back,” he said in Washington after the Pentagon briefed him on the status of the war.

“ISIL’s strategic weaknesses are real,” he added, using an acronym for the Islamic State.

But the absence of reliable local forces to press the fight deeper into the Islamic State’s home turf has revealed the weakness of the U.S. strategy, analysts say. And rising tensions between Arabs in the area and their purported Kurdish liberators risk jeopardizing the gains.

The offensive is taking Kurdish forces far beyond traditionally Kurdish territory and into areas where Syrian Arabs are in the majority, drawing allegations from Syrians and the Turkish government that the Kurds are taking advantage of the U.S.-led air war to carve out a Kurdish state.

The Syrian opposition has accused the Kurds of driving Arabs from their villages to consolidate their control.

“Their goal is to change the demography of the area and create a state of Kurdistan, and the reality is that this is happening under the cover of American airstrikes,” said Ahmed Haj Saleh, a longtime Syrian activist from Raqqa.

“I’m secular and I am an apostate, but if I have to, I will carry a weapon and join ISIS,” he said, using a term for the Islamic State. “I will not allow the demography of this area to change.”

Turkish President Recep Tayyip Erdogan has warned that his country will “never allow” the Kurds to establish a state on its border with Syria.

A spokesman for the Kurdish People’s Protection Units, or YPG, the Kurdish militia that is leading the fight, denied that the Kurds intend to establish a state. The areas captured “are a part of Syria and will remain part of Syria unless there is a decision by international powers,” YPG spokesman Redur Xelil said. Arabs who fled their homes are welcome to return unless there is evidence that they cooperated with the Islamic State, he added.

The tensions underscore one of the long-faulted shortcomings of the U.S.-led strategy to degrade and ultimately destroy the Islamic State — the lack of palatable alternatives to the Islamic State for the people living in its core territories, the Sunni Arab provinces spanning the borders of Iraq and Syria.

“That Sunni alternative simply doesn’t exist yet,” said Shadi Hamid of the Brookings Institution’s Center for Middle East Policy in Washington. “This is where the focus on quick wins becomes problematic. The U.S. isn’t thinking about what happens after ISIS is pushed out of an area.”

An unexpected victory

The recent battle in northern Syria turned into a quicker win than anyone expected.

An offensive to recapture the key border town of Tal Abyad, the Islamic State’s main conduit for foreign fighters and supplies from Turkey, was expected to take weeks at the least, U.S. and Kurdish officials said.

Instead, the Islamic State barely put up a fight. Its defenses collapsed last month after two days, and retreating fighters were quickly driven out of scores of towns and villages farther south, putting the advancing force within 35 miles of Raqqa.

Residents there reported signs of panic as the Islamic State dug trenches, appealed for volunteers in mosques, rounded up suspected dissidents and ordered thousands of Kurds in the city to leave.

“It’s like they are in shock,” said a businessman in Raqqa, who spoke on the condition of anonymity because he feared for his safety.

But airstrikes could prove counterproductive over the long term if they are viewed as empowering groups alien or hostile to the local population, analysts say. The YPG has been an effective U.S. partner in the war, but it is currently the only one in Syria, making it difficult to press the advantage of the militants’ apparent disarray.

A $500 million Pentagon program to train and equip moderate Syrian rebels to confront the Islamic State has barely gotten off the ground, more than a year after Obama announced it. Fewer than 200 rebels have entered the program, and U.S. officials say they are having a hard time finding Syrians who are willing to prioritize the fight against the Islamic State over the battle to unseat Syrian President Bashar al-Assad.

Although Free Syrian Army units are fighting with the Kurds, they represent a small minority of the overall force, which is grouped in a coalition called the Burkan al-Furat, or Euphrates Volcano.

The biggest of the rebel brigades, Thuwar al-Raqqa, or Raqqa Revolutionaries, wants to press on to Raqqa city, said Abu Shujaa, a spokesman for the group, who uses a pseudonym. But unlike the Kurds, who coordinate airstrikes through a U.S. operations center in the neighboring Kurdish region of Iraq, the Raqqa Revolutionaries have no contact with the U.S. military and therefore no way to call in the airstrikes that proved so crucial to the recent conquests.

The Kurdish fighters are more interested in turning west, the spokesman said, toward the Islamic State-controlled border town of Jarablus, to further expand the Kurdish enclave, rather than in advancing toward Raqqa.

“It seems the coalition doesn’t trust Arabs. They only bomb ISIS to help the Kurds,” he said.

Obama acknowledged that more must be done to train and equip local forces and that ultimately defeating the militants will be “the job of local forces on the ground.”

The Islamic State “has filled a void, and we have to make sure that as we push them out, that void is filled,” he said.

U.S. officials say they are looking for ways to team with rebel groups to take the fight deeper into Syrian Arab areas, including Raqqa. “The key will be the Arab units, and we’re very prepared to work with them and strengthen them wherever we can,” said a senior administration official, who spoke on the condition of anonymity to discuss strategy.

Such partnerships would offer an alternative to the U.S. plan to train and equip a separate Syrian force, but finding those groups “is itself a challenge,” Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told a congressional hearing last month.

Risk in Kurdish gains

The lack of partners in Syria is in part the fault of the administration’s “Iraq first” strategy, which has prioritized the fight in Iraq over that in Syria, said Hassan Hassan, who co-wrote the book “ISIS: Inside the Army of Terror” and is an analyst at Chatham House, a London-based think tank. He has long maintained that Syria may prove an easier battleground than Iraq, where the Islamic State has a long history and offers Sunnis an alternative to the Shiite-dominated government in Baghdad.

“ISIS is far less entrenched in Syria than in Iraq. It’s new to Syria, it’s a foreign organization, it’s more like an Iraqi organization than a Syrian one,” Hassan said.

“In Syria, there’s more potential, because there are more forces on the ground, but they are not given enough help,” he added, citing the presence of numerous rebel groups that have been fighting the Islamic State for more than a year.

That could change, however. Much in the way that Iraq’s reliance on Kurdish and Shiite fighters has alienated Sunnis there, Kurdish gains in Syria risk driving support for the Islamic State, said Hamid, of the Brookings Institution.

“This is exactly what ISIS wants. They want to be perceived as the last line of defense for Sunnis in Iraq and Syria,” he said. “The extent to which we contribute to that narrative becomes really problematic.”

The narrative already has taken hold among the thousands of Syrians who fled the recent fighting across the border into the Turkish town of Akcakale. Many say they won’t return as long as their homes remain under the control of the YPG, whose leftist ideology is at odds with the conservative inclinations of many from the area.

“These new guys have no mercy,” said a 22-year-old man who asked to be referred to only as Abu Mohammed because he is concerned about his safety. He fled ahead of the rapidly advancing Kurdish force, but had the battle continued any longer, he would have joined the Islamic State, he said.

“If you want to defend your religion and your land and your honor, you should join Islamic State,” he said. “Because if you don’t, the YPG will come and take your land.”

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Missy Ryan in Washington contributed to this report.