The US-led coalition in Syria has shot down a Syrian military jet in Raqqa province.
The Syrian army said its warplane was on a mission against the Islamic State (IS) group when it came under fire on Sunday, according to state television.
It said the incident would have “dangerous repercussions” on efforts to fight terrorism.
The US said it acted in self-defence after the Syrian regime dropped bombs near US-backed fighters.
“In accordance with rules of engagement and in collective self-defence of Coalition-partnered forces [the plane] was immediately shot down,” said the US military in a statement.
The incident took place in the town of Ja’Din, which is held by the Syrian Democratic Forces (SDF) and lies on the outskirts of Raqqa.
The SDF rebel fighters, supported by US-led coalition air strikes, are encircling the IS stronghold of Raqqa.
Two hours before the plane was shot down, the US said forces loyal to Syrian President Bashar al-Assad attacked SDF fighters, “wounding a number” of them and driving them from Ja’Din.
The US said it responded without hesitation, but emphasised that it did not seek to fight to the Syrian regime.
“The demonstrated hostile intent and actions of pro-regime forces toward Coalition and partner forces in Syria conducting legitimate counter-Isis operations will not be tolerated,” it added.
Syrian pro-government forces are not fighting the battle for Raqqa, but they are making ground against IS in the surrounding area, south-west of the city.
Earlier this month, the US shot down a pro-Syrian government armed drone after it fired at coalition forces near the al-Tanf border crossing, between Syria and Iraq.