Pentagon officials told The Daily Beast that the U.S. military was reassessing its air war over Syria after Moscow hinted it might shoot down American warplanes there.

"We're taking prudent measures to reposition over Syria," Defense Department spokeswoman Dana White told The Daily Beast. "We always take proper measures depending on the situation on the ground, but we're still operating… throughout Syria,” she said. “We always reserve the right to defend ourselves and our coalition partners."

Over the weekend, the U.S. took out a military jet belonging to the Syrian military, a Russian ally. That prompted Moscow to warn that “the destruction of the aircraft of the Syrian Air Force by the American aviation in the air space of Syria is a cynical violation of the sovereignty” and an “an act of military aggression against the Syrian Arab Republic,” the official statement posted on social media read.

The ministry said it also cut the phone line it uses to deconflict Syrian military operations with the U.S.-led coalition following Sunday’s incident.

Some U.S. military officials responded with a studied indifference, saying they are currently flying west of the Euphrates, and the deconfliction line is still working as far as their aware.

“We’ll continue to provide support for our coalition partners and our coalition partner forces are on the ground,” wherever needed, said coalition spokesman Col. Ryan Dillon from Baghdad.

That could put them in the crosshairs of Russian anti-aircraft systems, according to the Russian statement, which warned that “in the combat mission zones of Russian aviation...all kinds of airborne vehicles, including aircraft and UAVs [unmanned aerial vehicles, or drones] of the international coalition detected to the west of the Euphrates River will be tracked,” by Russian surface to air missile systems.

But Pentagon officials did indicate they were changing their air operations over Syria. They refused to elaborate, however, on whether that meant they were avoiding Russian combat areas or beefing up air patrols, fearing U.S. allies forces and the American military advisors with them might be facing a greater threat.

A former senior defense official said the U.S. military has to say, in public, that it reserves the right to fly anywhere in Syria. But its actions in the coming days will determine whether the Euphrates River is shaping up to become a de facto border between U.S. and Russian flight operations. If any U.S. warplane flies into areas patrolled by Russian jets any time soon, it’s likely to be a drone, so no one dies in the event the Russians or Syrians attempt a shootdown.

“They may lower the risk for awhile and send more unmanned systems where possible and generally fly in ways that aren’t mistaken as a provocation,” the former official said.

“So de jure, the U.S. military will fly anywhere, but de facto, we may—at least for a while—fly mostly in safer areas.”

The latest U.S.-Russian tensions began on Sunday, after the American military said its Syrian allies, the Syrian Democratic Forces (SDF), came under attack by Bashar Assad’s air force south of Tabqah. American warplanes engaged in what U.S. Central Command called a “show of force” to end the assault. According to a CENTCOM statement, U.S. military officials called their Russian interlocutors over the deconfliction channel to get them to rein in Assad. But two hours after the initial engagement, a Syrian SU-22 again attacked the US-backed force, prompting a US F/A-18 Super Hornet to shoot the jet down “in collective self-defense of Coalition-partnered forces.”

The town near Taqbah, Ja'Din, is only two kilometers north of what CENTCOM’s statement called an “established East-West SDF-Syrian Regime de-confliction area.” To some observers, the increasingly close proximity of all the various participants in the Syrian civil war—Assad, Russia, ISIS, the U.S.-led coalition, anti-Assad rebels, Turkey, Iran, Iranian-backed forces—is carrying with it a heightened prospect of new sub-conflicts, either by design or by accident.

Among them: the likelihood that the U.S. and Iran or their various proxies might end up aiming their weapons at each other—something that appeals to White House hardliners.

“As all sides get closer together, the decision to get more aggressive with the Iranians has consequences. They [the White House] either want more of a confrontation or they are blind to the risks of escalation,” said the former senior defense official.

White said the deconfliction channel is still working as far as she knows, and the Americans are willing to use it. “The Russians have been helpful with mitigating situations on the ground. I can't speak to what the Russians are going to do or have done,” she said, adding that the Pentagon got no official notice of a change in Russian military operations.

Dillon said the U.S. was “always available to deconflict with the Russians.”

The deconfliction telephone line, based at al Udeid Airbase in Qatar, worked when the Americans and Russians did their daily check-in Monday, a U.S. official said. “I guess we’ll find out tomorrow if they decide not to answer,” the official said, speaking anonymously because he was not allowed to describe coalition military communications publicly.

Russian officials in Washington declined to comment. But, contradicting CENTCOM’s statement, the Russian defense ministry claims the U.S. did not use the deconfliction channel with Russia before shooting down the Syrian warplane.

“We are always available to deconflict with the Russians,” said Dillon in comments to The Daily Beast from Baghdad. “It’s proven useful in the past, useful in de-escalating situations in the past. We remain available to use it in future,” he said.

Russian officials in Washington, D.C., declined to comment.

U.S. Central Command said Sunday that a Syrian jet had bombed its Syrian Democratic Forces allies south of the Syrian town of Tabqah , triggering the shootdown.

According to a statement, the U.S. military had asked Russia via the deconfliction line to warn the pro-regime forces to back off ahead of the incident, but they’d continued to attack. The Russian defense ministry claims the U.S. did not use the deconfliction channel with Russia before shooting down the Syrian warplane.

Statement of the Russian Defence Ministry concerning downing of the Syrian Su-22 near the town of Resafa

On June 18, 2017 the American fighter F-18A belonging to the international coalition shot down the Su-22 aircraft of the Syrian Air Force, which was performing a combat mission supporting the government troops, which were conducting the offensive against the ISIS terrorists near the town of Resafa (40 km to the south-west of the city of Raqqa).

As a result of the attack, the Syrian aircraft was destroyed. The pilot baled out over an ISIS-controlled area, his status is unknown.

The destruction of the aircraft of the Syrian Air Force by the American aviation in the air space of Syria—is a cynical violation of the sovereignty of the Syrian Arab Republic.

Numerous combat activities of the US aviation carried out under the cover of “fight against terrorism” aimed against the legitimate Armed Forces of a UN-member is a blatant breach of the international law and is in fact an act of military aggression against the Syrian Arab Republic.

Moreover, at that time the aircraft of the Russian Aerospace Forces were also performing combat missions in the air space of Syria. However, the Command of the coalition forces did not use the existing channels of communication between the Command of the Al Udeid Air Base (Qatar) and the Hmeymim Air Base Command to prevent air incidents in the air space of Syria.

The Russian party considers those actions of the US Command as an intentional failure to fulfill its obligations within the Memorandum on prevention of incidents and providing of flight security during the operations in Syria dated October 20, 2015.

Since June 19, 2017, the Russian Defence Ministry has stopped the cooperation with the American party within the Memorandum on prevention of incidents and providing of flight security during the operations in Syria and demands a thorough investigation of the incident by the US Command with further providing of information on its results and the taken measures.

In the combat mission zones of the Russian aviation in the air space of Syria, all kinds of airborne vehicles, including aircraft and UAVs of the international coalition detected to the west of the Euphrates River will be tracked by the Russian SAM systems as air targets.