Washington is loath to cooperate with Assad's internationally ostracized government. But it will be difficult to uproot ISIS militants and keep them out with only the Kurdish and Arab militias backed by the US—and a coalition spokesman pointed out that Assad's gains ease the burden on those forces.
Letting Assad grab ISIS territory, however, risks being seen as the US legitimizing his continued rule and would likely strengthen his hand in his war against the already struggling rebellion. It also threatens to further empower Assad's allies, Iran and the Lebanese Hezbollah, which both have forces alongside his troops in the assault into ISIS-held territory.
Within the Trump administration, there is a split over whether to aggressively try to stem Assad's advances, said a senior US official, who wasn't authorized to speak to reporters and requested anonymity.
Army Col. Ryan Dillon, the spokesman for the anti-ISIS coalition, said Syrian government forces are welcome to reclaim ISIS-held territory and fill the vacuum once the extremist group is gone.
The statement was startling—even more so because soon after President Donald Trump this week warned Assad he would pay "a heavy price," claiming "potential" evidence that Syria was preparing for another chemical weapons attack.
The mixed messages reveal a discomfiting fact that most policy makers would rather not spell out: Assad is a pariah but he is also a convenient tool to secure and govern territory in majority-Arab cities in a complex terrain.
The situation in Syria is a contrast to Iraq, where the coalition and the Iraqi government, working hand in glove, appear to be on the verge of retaking the main ISIS redoubt in city of Mosul.
The Syrian government has repeatedly suggested that everyone is welcome to work with it to defeat ISIS.
Mohammad Kheir Akkam, a Syrian lawmaker, questioned US support for the Kurdish-led forces "despite the fact that the Syrian-Russian cooperation has achieved more results in combating terrorism," while US efforts have "had the opposite result."
The US so far has shunned any cooperation with the Syrian leader, whom Trump described as an "animal." Instead, it has partnered with local Kurdish and Arab forces known as the Syrian Democratic Forces, or SDF.
Those fighters are currently spearheading the assault on ISIS' self-declared capital, Raqqa in northern Syria, and then face the prospect of assaulting the group's final major stronghold to the southeast, in Deir el-Zour.
But US support for the Kurdish-led group has angered Turkey, which views it as an extension of a Kurdish insurgency within its own territory. The SDF is also viewed with suspicion by the predominantly Arab residents of Raqqa and Deir el-Zour.
Furthermore, the SDF, numbering around 50,000 fighters, is already risking overstretch and is in no way ready for the more challenging battle in Deir el-Zour.
Assad and his Iranian allies, on the other hand, have steadily positioned themselves in key areas on the flanks of the US-led war against ISIS, grabbing territory on several fronts, including on the outskirts of Raqqa and Deir el-Zour. With Russian and Iranian support, Assad has made steady gains and now controls almost all of Syria's major cities except those held by ISIS.
The symbolism was striking this week as a smiling Assad paid a visit to central Hama, driving his own car, and to a Russian air base in western Syria, where he posed alongside Russian generals and inside the cockpit of a Russian SU-35 fighter jet.
Syrian troops have positioned themselves on Raqqa's southwestern flanks, and officials have vowed to retake the city eventually.
The US has insisted that the city should be handed over to a local council that would handle its administration post-liberation—and it has made clear it will not tolerate the Syrian government and its allies cashing in on the fight. US forces recently shot down a Syrian aircraft as well as drones believed connected to Iranian-supported forces as tensions escalated near a base where the coalition trains Syrian rebels.
But the senior American official said there was significant disagreement about how aggressively the US should try to prevent Assad from reclaiming the territory ISIS vacates, with some in the White House pushing a more forceful approach while the State Department and the Pentagon warn of the risks.
Keeping Assad's territory to a minimum would ensure his hand isn't strengthened in an eventual political deal to end the conflict, making it more likely the US could deliver on its longstanding desire to see him leave power. Limiting his control in eastern Syria would also prevent Iranian-backed forces from securing a wide corridor through Iraq to Syria and all the way into Lebanon.
The more risk-averse voices in Trump's administration are wary about letting the US slip into a more direct fight with Assad, the official said.
Dillon, the coalition spokesman, told reporters at the Pentagon that the US goal is to defeat ISIS wherever it exists. If others, including the Syrian government and its Iranian and Russian allies, want to fight the extremists, "we absolutely have no problem with that."
Frederic C. Hof, director of the Atlantic Council's Rafik Hariri Center for the Middle East, said the comments reflect the narrow US view of the Syria war, focused very specifically on the neutralization of ISIS.
In the coalition view, "It is all about killing ISIS in Raqqa." Hof wrote in an article this week. "Creating conditions that would keep it dead? That, presumably, would be someone else's job."