On Sunday afternoon the US-led coalition fighting Isis blocked pro-Assad forces from entering the Syrian Democratic Forces (SDF)-held town of Ja’Din in Raqqa province.

And then they called the Russians.

It’s called the de-confliction hotline. Imagine a wire stretching from the Persian gulf to the eastern Mediterranean with one tin can at the US Air base at Al Udeid in Qatar and the other at Russian-rented Khmeimim air base in Western Syria.

The line exists mostly to prevent confusion between the two superpowers on what the other is doing in the region.

Before the US fired 59 Tomahawk missiles at Shayrat Airfield, for example, they hopped on the line to let Russia know it was coming.

Chris Woods, director of Airwars, which tracks harm to civilians on the ground, described the hotline mainly as being there to prevent Russian, Syrian, US and other aircraft from crashing into each other.

It is a very densely packed airspace with a lot of competing factions. So the hotline is partly to ensure that aircraft don’t collide with each other, errors aren’t made that then lead to skirmishes and so that the sides can alert each other when their forces wrongly come under fire (from) the other side.

Woods says while there is clearly value in having a hotline in place he doesn’t think it has ever done a great deal to protect civilians from airstrikes.

This is really about pilots not killing each other, very little to do with civilians on the ground.

The line also functions as a way of easing tensions after skirmishes, as the various forces inevitably encroach on each other’s territory. That’s why the Americans called up on Sunday.

“Following the Pro-Syrian forces attack,” a statement read, “the coalition contacted its Russian counterparts by telephone via an established de-confliction line to de-escalate the situation and stop the firing.”

Unfortunately these efforts fell short and the engagement continued, reaching a peak late in the afternoon when according to the coalition:

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A Syrian regime SU-22 dropped bombs near SDF fighters south of Tabqah and, in accordance with rules of engagement and in collective self-defence of coalition partnered forces, was immediately shot down by a US F/A-18E Super Hornet.

In other words the US say they shot down a Syrian bomber for attacking coalition allies.

The Russians say the bomber was operating in support of a government anti-Isis offensive and the US were out of line for getting involved.

“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.” read a statement on the Russian Ministry of Defense Facebook page.

They said the action is a blatant breach of international law and constitutes an act of military aggression against the Syrian regime.

And now they’ve said they’re cutting off the hotline.

(As of) 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.

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 (surface-to-air missile) systems as air targets.

That is to say Russian forces will no longer warn the US when they are carrying out exercises and will treat coalition aircraft as potentially hostile.

Or at least that’s what they seem to be saying. Airwars’ Chris Woods says the signals coming from Russia are somewhat ambiguous and it is unclear how much consequence this statement actually holds.

But he said the threatened ending of the de-confliction hotline is something to worry about nonetheless.

Should aircraft collide, should they stray into each other’s territory, the risk of things then getting out of hand, even accidentally, is made worse and that’s something nobody wants.

The main issue here according to Woods is that there are two major international powers, each with a significant number of aircraft and weaponry as well as perhaps a dozen other international belligerents, all conducting strikes at various times in a very cramped space.

It is a fraught and fragile environment and things can go wrong and previously have gone wrong. Tensions are clearly rising, and these skirmishes are intensifying between the US led-coalition and the Syrian government-led coalition.

That is bad news for everybody. And meanwhile on the ground lots of civilians continue to die.