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The increasing number of close calls between US and Russian aircraft over Syria is a worrying development which is showing great potential to lead to an accident or misunderstanding with deadly consequences.

On December 13, two United States Air Force (USAF) F-22 Raptor jet fighters intercepted Russian aircraft flying very close to them. The Raptors shot flares in order to warn off the two Russian Su-25 Frogfoot jets.

The Russian aircraft were reportedly operating on the wrong side of the deconfliction line over the Euphrates River which the US-led coalition had earmarked, and which the Russians agreed to abide by, to give the separate coalition and Russian campaigns against the Islamic State group in eastern Syria two clear areas in which to conduct their operations.

What makes this incident so worrying is the fact it is not isolated. For well over a year, erratic Russian flying has worried the US. Lack of coordination could result in a misunderstanding which could quickly lead to a mid-air collision or even a shootdown.

Last January, The Wall Street Journal detailing how US pilots are fearful of the eventuality of a mid-air collision with their Russian counterparts in the skies over Syria.

The article describes an incident in the autumn of 2016 when a Russian Su-35 Flanker rapidly approached a US radar plane and came within a few hundred feet - before it flew "north and east across the American plane's nose, churned up a wave of turbulent air in its path and briefly disrupted its sensitive electronics".

The article went on to add that more broadly "the skies above Syria are an international incident waiting to happen, according to American pilots".

The fact that "the Russians don't emit identifying signals, flouting international protocols" makes the risk more great.

[In] one incident on November 15... two USAF A-10 Warthog attack planes nearly collided with a Russian Su-24 Fencer bomber which flew within 300 feet of the American planes

It has been a year since that article's revelations and things seem to have only been getting worse. A mere week before this month's incident with the F-22, The New York Times detailing the erratic flying of Russians in the preceding month.

It described one incident on November 15, in which two USAF A-10 Warthog attack planes nearly collided with a Russian Su-24 Fencer bomber which flew within 300 feet of the American planes on the American side of the aforementioned Euphrates deconfliction line.

Over the next two days "a Russian Su-30 Flanker flew 1,000 feet directly below A-10s". Additionally, on November 17 two USAF F-22s "encountered an armed Russian Su-24 Fencer that had crossed into the airspace east of the Euphrates, and made three passes directly over allied ground forces for 20 minutes".

Read more: US jets 'intercept Russian warplanes' in Syria

Pentagon officials claim the Russians are intentionally crossing the Euphrates line and sometimes flying "within striking distance or directly over allied ground forces for up to 30 minutes, escalating tensions and the risk of a shootdown".

The fact that, in many of these incidents, the Russians did not reportedly even establish communications with the Americans means the Pentagon could have justified shooting them down - given that their behaviour could constitute a clear threat to American forces in the air and on the ground.

The USAF engaged in its first air-to-air shootdown in Syria over the summer - against a Syrian bomber targeting Washington's Syrian Democratic Forces (SDF) allies battling IS in Tabqa. 

USAF F-15s also shot down Iranian drones flying near a US base of anti-IS fighters at Al-Tanf on the border with Jordan on two occasions.

Russian Su-34s attacked that same base twice with cluster bombs in June 2016 and claimed, unconvincingly, that it was an accident. On that occasion US Navy F/A-18 Hornet jet fighters to intercept their Russian counterparts.

And, in August 2016, USAF F-22s were promptly scrambled to intercept Syrian Su-24 bombers over the city of Hasakah, after they bombed Syrian Kurdish forces close to a US commando position in the vicinity. The Syrians rapidly pulled out of the area.

All these incidents demonstrated how quickly the US can deploy its fighter aircraft in the region to defend positions across Syria.

Only one minor mistake made under intense pressure could have drastic consequences

These repeated Russian efforts to challenge US jets by flying close to them and gauging their intercept time, through often erratic and reckless flying - a recent video also shows a Russian Su-30 flying dangerously close to a Russian cargo plane in a stunt to impress its crew, showing that the unsafe flying of the Russians over Syria is not just restricted to testing the resolve of the Americans - increases the risk of one fatal accident that could have serious ramifications.

The Turkish shootdown of the Russian Su-24 bomber over its border in September 2015 (the first shootdown of a Russian warplane by a NATO air force in half a century) aptly demonstrated this danger. While the US has clearly shown much more restraint than the Turks did on that occasion, only one minor mistake made under intense pressure could have drastic consequences. The frequency of Russia's erratic flying and transgressions only increases this deadly danger.

At this late stage in the Syrian war, as the American-led and Russian campaigns in the country are coming to an end, it's in neither side's interests to risk reaching that point. 

Paul Iddon is a freelance journalist based in Erbil, Iraqi Kurdistan, who writes about Middle East affairs.