Washington and Damascus are not coordinating their battle plans against the so-called Islamic State at any official level, but a de facto deal between them is increasingly obvious. The Assad regime is conducting follow-up bombing raids in the wake of sorties by the U.S.-led coalition, and it has launched a land offensive in eastern Syria that is helping the coalition and the Kurds shut down the jihadist supply lines to Mosul in northern Iraq. This evident, if indirect coordination, is feeding Sunni Muslim suspicions in the region that Syrian President Bashar Assad and U.S. President Barack Obama have decided to work together.

The Syrian civil war, now entering its fifth year, has witnessed a dizzying sequence of shifting alliances. Enemies suddenly become friends and allies are deemed foes in a real-life Game of Thrones. So it’s not surprising that Western-backed rebels are uneasy about recent developments.

Their suspicions are being stoked by the toned-down anti-Assad rhetoric of Obama officials. In his State of the Union last month, Obama focused his Syria remarks on the so-called train-and-equip plan to build a proxy force that will target Islamic State militants. And also by Assad himself.

In an interview with the BBC, broadcast Tuesday, the Syrian tyrant stated flatly that Washington is sharing information on coalition airstrikes via intermediaries. Asked specifically why there haven't been any incidents such as mid-air collisions or exchanges of fire between Syrian and coalition warplanes, Assad said there are ongoing communications “through a third party—more than one party—Iraq and other countries.”

Assad added: “Sometimes they convey a message, a general message, but there's nothing tactical.” He said there had been no direct cooperation since coalition air strikes began in Syria in September.

But, even if we take this at face value, actions by the Syrian armed forces in parallel with coalition airstrikes are affecting the critical ability of ISIS to move its fighters quickly over the region’s desert terrain and depriving it of the chance to compensate for battlefield setbacks in one theater by switching focus to one side or the other of the Syrian-Iraq border.

“We are seeing coalition warplanes hit targets during the day in Raqqa province and then Syrian warplanes follow-up with more indiscriminate strikes at night,” a commander with the Free Syrian Army told The Daily Beast. “This is not a coincidence—to argue that it is stretches credulity.”

At least as important in the ground war are the clashes under way between pro-Syrian-government militias and Islamic militants in northeast Syria. Those skirmishes are helping Iraqi Kurdish Peshmerga fighters squeeze jihadist supply lines into the northwestern Iraqi city of Mosul.

On the Syrian side of the frontier the fighting is led by a branch of Assad’s military known as the National Defense Forces, or NDF, whose members are drawn largely from Syrian minorities threatened by the savage absolutism that the self-proclaimed caliphate imposes in the name of Sunni Islam. The NDF has been built up with the support of Assad’s ally Iran and is modeled on Tehran’s own Basij militia.

Last month, Kurdish fighters seized a string of villages and a key highway intersection between Mosul and the nearby Iraqi town of Tal Afar. The rollback has continued with Kurdish forces, backed by coalition airpower, making new gains around Mosul and seizing control over three strategic areas near the city, the U.S. Central Command said in a statement on Monday.

“Security Forces from the Kurdish region seized three bridgeheads on the west bank of the Tigris River, north of Mosul in formerly held Daesh [ISIS] areas,” the statement said. The Kurdish play was helped by “four close air support airstrikes to facilitate the maneuver of the Kurdish Security Forces and their successful attack.”

ISIS fighters started to respond last month to the Kurdish gains by probing territory southeast of the long-besieged city of Kobani, which was defended successfully and finally re-taken by Syrian Kurdish fighters. According to Western officials, ISIS appeared to be massing forces for an offensive on the town of Hasakah, a vital crossroads about halfway between Mosul, the second largest city in Iraq, which was taken by ISIS last summer, and Raqqa in northern Syria, which is the de facto capital of the putative Islamic State.

U.S. warplanes have launched airstrikes near Hasakah in recent weeks and the NDF has stepped up its offensives against the ISIS militants there as well.

This sticks in the craw of moderate rebel fighters. It’s not that they want ISIS to succeed, but they see all of this as a grand conspiracy by the wily Assad regime. They have long insisted that ISIS and al Qaeda affiliate Jabhat al Nusra were encouraged by Assad, who freed jihadist prisoners as the uprising against him began in 2011, knowing that would help him brand the entire opposition as extremist.

According to Michael Weiss and Hassan Hassan, co-authors of a new book, ISIS: Inside the Army of Terror, “Assad wasted little time guaranteeing that extremists dominated the insurgency,” a ploy intended to win Western support for the relatively “moderate” Syrian regime.

The rebels argue that the tactical objectives of Assad and the Islamic State are overlapping: both fuel sectarianism by attacking enemy sects, provoking reprisals and polarization and thus shoring up their own sectarian support.

That may well be true. But whatever conspiracies lay behind the actions of the Assad regime two or three or four years ago, the fact is the landscape changed dramatically after the ISIS offensives last summer conquered huge swathes of both Syria and Iraq.

The informal coordination between Washington and Damascus appears to be aimed at halting the momentum of ISIS and disrupting its supply lines in anticipation of a major offensive on the Iraqi side of the frontier around Mosul some time in the next few weeks. That certainly will not end the war, but it may begin to turn the tide of battle.

What comes next in this treacherous game of blood and deceit is anybody’s guess.