American-backed Kurdish and Arab forces have found an unlikely ally in their push to win ground in Syria: the Russians.

Some of the nearly 25,000 U.S.-backed forces made some of their biggest gains in Syria in recent months when they retook villages near the hotly contested city of Aleppo with the help of Russian airstrikes. The Russian attacks targeted other opposition groups, and the U.S.-backed coalition exploited them to gain back parts of the city.

The Russian strikes were not designed to help those U.S.-backed forces, nor were the groups coordinating with the Russians. But nevertheless, the air attacks allowed the U.S.-created Syrian Democratic Front to make advances, four defense officials told The Daily Beast.

“It’s not a planned thing. It is not something Russians are trying to do. It is happenstance,” one U.S. official told The Daily Beast. “The net sum of everything the Russians are doing helps Assad and prolongs the war.”

And now, U.S. officials fear such success could create a new alliance between its proxy forces in Syria and the Russian military, which American generals have repeatedly called a potential “existential” threat to the United States.

The Russian strikes could appeal to the Kurds because the U.S.-led coalition’s air war is aimed almost exclusively at the self-proclaimed Islamic State, whereas the Russians have attacked any group opposed to Assad. The Kurds and Syrian rebels want to see Assad toppled, as well as ISIS defeated.

Officials believe that within days of Russian airstrikes on positions held by the Nusra Front, al Qaeda’s branch in Syria, Kurdish forces moved forward to take at least three villages within the city, helping them make notable gains.

Aleppo, Syria’s second-biggest city, has long been a battlefield, and a prize for nearly every group vying to control territory in Syria.

There is no indication of direct communications between the American-backed ground forces and the Russians, U.S. officials said. But the fact that the Kurds made so much progress in Aleppo thanks to those Russian airstrikes has some U.S. officials worried that a new Kurdish/Russian alliance could form. That would add another layer of complexity in an already fraught war in which alliances are constantly shifting.

The Russians, after all, are committed to saving the Bashar al-Assad regime. The U.S. is formally committed to ending it. And so far, the Obama administration has resisted all calls to put aside those differences and form some sort of grand alliance with the Kremlin.

“Coordination with the Kurds would allow for Russia to continue to present itself as a worthwhile anti-ISIS actor in Syria, while angering Turkey and disrupting U.S. operations,” Genevieve Casagrande, a research assistant at the Institute for the Study of War who focuses on Syria, told The Daily Beast. “We aren’t seeing this escalation quite yet, but it is something that these airstrikes can open up the possibility for in the future.”

Two U.S. officials told The Daily Beast they have seen a similar pattern of ground forces moving in following Russian airstrikes near Raqqa, the self-proclaimed Islamic State capital. These land grabs weren’t as successful as those in Aleppo, officials said.

Nor do they necessarily represent a harbinger for a future in which Kurdish forces overtly side with the Russians—a development that could potentially undermine the Kurds’ alliance with the U.S. and strengthen Russia’s influence.

Two U.S. officials told The Daily Beast that the rebels and the Kurds weren’t likely to take sides with the Russians, at least not in the near term. Their alliance with the U.S. has largely been one of battlefield convenience. But the ground forces have a longer history with the Americans that they’re not likely to abandon overnight, the officials argued.

Still, a Kurdish and rebel alliance with the Russians can’t be ruled out. Indeed, in the complex stew of the Syrian conflict, various factions have coordinated with their adversaries.

“If you look at the entire course of fighting on the ground, it’s an enemy-of-my-enemy-is-my-friend war,” Daveed Gartenstein-Ross, a senior fellow at the Foundation for the Defense of Democracies, told The Daily Beast. The YPG—the Syrian-Kurdish military forces—has coordinated with the Assad regime, and the Free Syrian Army has done so with al Nusra, he said. In that context, it’s not only plausible that U.S-backed and allied groups would coordinate with the Russians; it’s likely.

Recent events could also be driving the Kurds closer to Moscow. After Turkey shot down a Russian jet it said had crossed into its airspace last year, Russians took to social media to express sympathy for and solidarity with the Kurds in their conflict with Turkey.

Also, in November, the Kurdish Democratic Union Party began meetings with Russia to discuss its own plans for greater self-rule. That was an apparent stab at the U.S., Russia’s strategic rival, and arguably a bid to win Washington’s support for Kurdish independence, as well as more weapons.

One more reason the Kurds might saddle up with Moscow: They wouldn’t necessarily see their new alliance as turning their backs on the U.S. The Russians are conducting more strikes and have less compunction than the American-led coalition about whom they target.

The result has been a terrible human toll. Most recently, the Syrian Observatory for Human Rights alleged Russian strikes in Aleppo killed at least 12 children and three adults.

But the Kurdish rebels may be using a different calculus.

“I think from their perspective, there’s no reason that they wouldn’t work with the Russians and with us,” Gartenstein-Ross said. “The Russians have at this point a more robust presence. I’m not sure they’d see it as betraying their relations with the U.S.”