Female fighters with the U.S.-backed Syrian Democratic Forces celebrate Oct. 19, 2017, in Raqqa, Syria, beneath a banner of Abdullah Ocalan, the imprisoned leader of the militant Kurdistan Workers’ Party, which has been designated a terrorist group by Turkey and the United States. (Bulent Kilic/AFP/Getty Images)

Rapid advances by Russian- and Iranian-backed government forces in eastern Syria are thwarting the U.S. military's hopes of pressing deeper into Islamic State territory after winning the battle for Raqqa.

An expansion of territory held by forces loyal to President Bashar al-Assad is also likely to provide Assad with additional leverage in political negotiations over Syria's future, talks the United Nations hopes to reconvene next month.

In a statement this week, U.N. Secretary General António Guterres said the "latest developments" in Syria pointed "to the urgent need to reinvigorate the political process."

The recent government gains have cut off the approach of the U.S.-backed Syrian Democratic Forces to remaining militant strongholds in the southeastern part of the country, including the crucial town of Bukamal near the Syria-Iraq border.

Aided by Russian airstrikes, in apparent violation of a deconfliction line along the Euphrates River that U.S. officials said had been tentatively agreed on with Moscow, government forces have encircled and claimed control of another location that had been on the wish list of U.S. military planners — the town of Mayadeen, where many senior Islamic State leaders are thought to have been hiding. The militants put up little resistance, and most appear to have escaped.

[Graphic: The rise and fall of the Islamic State in Iraq and Syria]

The unexpected militant withdrawal has "thrown for a loop" U.S. military assumptions that it could beat overstretched government forces in a race to the key river strongholds, said Nicholas Heras, a fellow at the Center for a New American Security. "Because ISIS has decided not to put up a tough fight against Assad's forces," Heras said, "it has forced a change of assumptions about what the situation will look like on the ground."

The advance has also taken government forces, and supporting Russian strikes, east of the river and into Syria's main oil-producing region of Deir al-Zour province, once a key source of Islamic State revenue.

"I'm not going to address whether or not an agreement or deconfliction line has been broken," Army Col. Ryan S. Dillon, spokesman for counter-Islamic State military operations, said in a telephone interview from Baghdad. "That's why we maintain an open dialogue" with Russia.

In addition to daily contact between the two militaries on a hotline, U.S. and Russian generals have held two face-to-face meetings in recent weeks, at least one of them in Jordan, to discuss the increasing proximity of their air operations in the Euphrates River valley, and that of the separate ground forces they back.

Progress against the Islamic State in Syria has been measured since 2016 by towns and cities seized from militant control along the Euphrates by the SDF, a combination of Arab and Syrian Kurdish fighters, aided by U.S. air power and advisers. Manbij, near the Turkish border in the north, was recaptured in 2016, followed by Tabqa and now Raqqa.

After Raqqa, the intention was to proceed downriver through Mayadeen to Bukamal, where SDF fighters would link up with Iraqi government forces trying to regain control over the Islamic State-controlled town of Qaim, just across the border inside Iraq. A major goal was to block Iran from securing a land corridor, through Iraq, between Tehran and Damascus.

Dillon declined to say whether the U.S. military's plans had changed.

"There are always plans," Dillon said. "You don't fight the plan, you fight the enemy . . . where they are." The military, he said, was not concerned with "greater policy decisions" over who fought the militants or who controlled Syria, as long as it was not the Islamic State.

"We're not in a race, we're not in the land-grab business. We're here to defeat ISIS," he said, using an acronym for the Islamic State.

Others were less sanguine about the effect of government gains, predicting that Assad's ability to remain in power would leave open the door for Islamic State militants, gone to ground in the vast desert that spans the Syria-Iraq border, to regroup.

"That's what you get when you make a deal with the Russians," said Jennifer Cafarella of the Washington-based Institute for the Study of War, which monitors the fighting in Syria. "What we see is a push by the regime and its backers to seize key infrastructure, such as oil and gas fields, and to position to disrupt U.S.-led anti-ISIS operations further down the Euphrates."

With the remaining Islamic State strongholds in Syria increasingly likely to fall into ­Syrian government hands, the Trump administration will have to decide whether the U.S. military remains in Syria to protect areas that have been captured by the SDF — which is dominated by Syrian Kurds of the People's Protection Units, or YPG.

On Thursday, female YPG fighters marked the victory in Raqqa by raising a giant banner of the Kurdish leader Abdullah Ocalan over the central square where the Islamic State carried out most of its grisly executions. Ocalan, who heads Turkey's militant Kurdish movement, the Kurdistan Workers' Party, or PKK, is serving a prison sentence in Turkey for terrorism.

The public declaration of fealty to Ocalan by the Syrian Kurds who led the Raqqa offensive points to one of the many challenges confronting the Trump administration as it seeks to forge a coherent policy for the post-Islamic State era. Although the Syrian Kurds have admitted many Arabs into their ranks, they have retained overall control of the SDF coalition's command and ideology.

Turkey, which shares a long border with the autonomous enclave the Kurds have established in northeastern Syria, is enraged at the U.S. military's support for the SDF, which it considers an appendage of Ocalan's terrorist movement. That leaves the SDF vulnerable to potential military action by Turkey to quell its aspirations for a ministate in Syria.

Many Syrian Arabs are also deeply uncomfortable about the prospect of being governed by Kurds. Raqqa is an almost wholly Arab city, and the photographs of the Ocalan banner that circulated on social media triggered widespread condemnation by ­Arabs on Thursday.

"For us Raqqans, we do not know whether the SDF taking over the city and expelling ISIS is a liberation or an occupation," Tareq Sham, a former Raqqa resident living in Turkey, wrote on his Facebook page. "The vast majority of us consider what happened a switch between two occupiers."

Remaining in Syria to protect its Kurdish allies risks embroiling the United States in possible future conflicts between Arabs and Kurds, and between Turkey and the Kurds.

The Kurds are also vulnerable to the Syrian government's declared ambition to reclaim all of the territory it lost in the war that began as a political rebellion in 2011. Much of what happens in Raqqa will depend on the speed and success of reconstruction there. U.S. special envoy Brett McGurk is visiting the Raqqa area, accompanied by Saudi Arabian Minister for Gulf Affairs Thamer al-Sabhan, whose government the Trump administration hopes will put up funds for the effort.

Sly reported from Beirut.