Syrian government forces and US-led coalition are operating in close proximity in Raqqa province
An American fighter jet on Sunday shot down a Syrian warplane that the US-led coalition said attacked its allies in the fight against the Islamic State (IS) group in the war-torn country.
The escalation comes as Syria's six-year-old war becomes ever more complex, with US forces and their allies converging on the northern IS bastion of Raqqa in close proximity to Russian-backed government troops.
Further complicating matters, Iran said it launched missile strikes on Sunday against "terror bases" in Syria's northeastern Deir Ezzor province in revenge for deadly attacks on its capital claimed by IS.
The US-backed Syrian Democratic Forces - an alliance of Kurdish and Arab fighters - is battling to oust the militants from Raqqa, and broke into the IS stronghold city last week.
— ABC News (@ABC) June 18, 2017
Government forces are not involved in the battle for Raqqa, but they are advancing in an area southwest of the city, skirting around SDF fighters, their eyes set on the oil-rich province of Deir Ezzor.
"Aircraft from the 'international coalition' targeted one of our fighter planes in the Resafa region of southern Raqqa province this afternoon while it was conducting a mission against the terrorist Islamic State group," the Syrian army said.
It warned of "the grave consequences of this flagrant aggression".
The US-led coalition later confirmed it shot down the Syrian warplane that it said had dropped bombs near SDF forces.
"At 6:43 pm (1743 GMT), a Syrian regime SU-22 dropped bombs near SDF fighters... 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," the Combined Joint Task Force said in a statement.
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It said that two hours earlier, forces loyal to President Bashar al-Assad had attacked the SDF in Jaaydine, south of Tabqa, "wounding a number of SDF fighters and driving the SDF from the town".
Coalition aircraft stopped the advance of Syrian pro-government troops with a "show of force," the coalition added.
Before it downed the plane, the coalition had "contacted its Russian counterparts by telephone via an established 'de-confliction line' to de-escalate the situation and stop the firing."
The coalition does "not seek to fight the Syrian regime, Russian or pro-regime forces" but would not "hesitate to defend itself or its "partnered forces from any threat," the statement said.
The US-led coalition has in recent weeks escalated its aerial bombing campaign in northern Syria and Raqqa province.
The Syrian army said the "aggression" against it came as government troops and their allies made up ground in the battle against IS "on several fronts in the badiya".
It was referring to a large stretch of desert that extends over some 90,000 square kilometres from central Syria to the borders with Iraq and Jordan to the east and southeast.
Since 2015, much of the badiya has been held by the militant group, but Syria's army has been chipping away at it for months.
— Ted Lieu (@tedlieu) June 18, 2017
The Britain-based Syrian Observatory for Human Rights said the fighting broke out in two villages, Jaaydine and Shouwayhane, some 40km south of Raqqa.
The Observatory said the clashes came after government troops had reached the edges of Resafa, also in the same area, as part of an offensive to reach Deir Ezzor.
"The regime is trying to reach the oil province of Deir Ezzor through Raqqa," Observatory head Rami Abdel Rahman told AFP.
The regime has lost control of many oil and gas fields since the start in 2011 of Syria's conflict, especially in the provinces of Homs and Deir Ezzor.
Most of the province is held by IS, including parts of the provincial capital, Deir Ezzor city, and the militants are besieging government-held parts of the city.
SDF fighters entered Raqqa on 6 June after months of heavy clashes to encircle it, and the US-led coalition has been backing them with air strikes, equipment and special forces advisers.
Since then they have seized a few neighbourhoods, including one in the east and one in the west, and are battling to push into the Old City of Raqqa.
312 ISIS militants killed, four districts liberated in Raqqa operation so far: SDF https://t.co/Q16ZlL4N32
— Teoman Başbuğ (@TeomanBasbug) June 18, 2017
The Syrian army has also taken territory from retreating IS militants in the western Raqqa countryside and seized back some oil fields and villages that had been under the militants' control for almost three years.
An estimated 300,000 civilians were believed to have been living under IS rule in Raqqa, including 80,000 who fled there from other parts of the country.
IS seized Raqqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".
More than 320,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011 before turning into a complex war involving regional and international players.