The Pentagon is voicing growing alarm that the risky flying of Russian pilots in Syria could lead to a mishap -- or even the nightmare scenario of a US jet shooting down a Russian warplane.

Defense officials this week highlighted several recent close calls with Russian planes, including one Wednesday, when a pair of US F-22s intercepted two Russian jets over a part of Syria in which the Pentagon says they are not meant to be operating.

The uptick in incidents comes as remaining operations by the US-led coalition fighting the Islamic State group in Syria have shrunk down to an area of only about 15 square miles (39 square kilometers) around Albu Kamal in eastern Syria, by the Iraq border.

Coalition forces are giving air support to local Kurd and Syrian Arab partner troops on the ground as they root out remaining IS fighters east of the Euphrates River. Under a verbal agreement, the Russians, who support President Bashar al-Assad, are supposed to stay to the west.

Lieutenant Colonel Damien Pickart, an Air Force spokesman in the Middle East, outlined a string of instances where Russian fighter jets flew east of the Euphrates without notifying the coalition.

On November 15, two US A-10 Warthog ground-attack planes nearly collided head on with a Russian Su-24 Fencer that passed within only 300 feet (90 meters) of the American planes -- a mere whisker in aviation terms.

One A-10 pilot had to "aggressively execute a defensive maneuver to avoid a midair collision," Pickart said in an email to AFP.

Then on November 17, two F-22s intercepted an armed Russian Su-24 that flew over coalition and partner forces three times and failed to respond to radio call.

"The F-22s intercepted this pilot and were in a position to fire," Pickart said.

"Luckily our pilots showed restraint, but given the actions of the Su-24 aircraft could have reasonably been interpreted as threatening to US forces, our pilots would have been well within our rights to engage."

Defense Secretary Jim Mattis said it was not clear if the incidents were a mistake due to inexperience, or the product of boisterous young pilots "dangerously feeling their oats."

"I don't expect perfection, but I don't expect dangerous maneuvers either and so we'll sort this out," Mattis told Pentagon reporters Friday.

"Right now, I cannot tell you if it's sloppy airmanship, rambunctious pilots or people who are trying to do something that is very unwise."

- 'Greatest concern' -

Since Moscow entered the Syria war in late 2015, Russia and the United States have been using a special "deconfliction" hotline to communicate about operations occurring in similar locations. Officials use the line constantly.

A shootdown of a Russian jet, or a midair collision, could suddenly and dramatically shift the stakes in the knotted Syria conflict and open the door to a retaliatory measures by the Russians.

"The coalition's greatest concern is that we could shoot down a Russian aircraft because its actions are seen as a threat to our air or ground forces," Pickart said.

"We are not here to fight the Russians and Syrians -- our focus remains on defeating ISIS. That said, if anyone threatens coalition or friendly partner forces in the air or on the ground, we will defend them."

At one point during Wednesday's incident, the US F-22 Raptor stealth fighters deployed chaff and flares to convince the Russian Su-25s to leave the area, and one US pilot had to aggressively maneuver to avoid a midair collision, Pentagon spokesman Eric Pahon said.

During and following the encounter, coalition leaders contacted Russian officers on a special hotline to try to calm the situation and avert a "strategic miscalculation," Pahon said.

More than 340,000 people are estimated to have been killed in the Syrian war, and millions have been displaced.

Source: AFP