A USA military aircraft shot down a Syrian government fighter jet on Sunday shortly after the Syrian plane bombed US -backed fighters in northern Syria, the US military said.
Iran-backed Shiite militias, along with other pro-Syrian government forces, have steadily advanced around Tanf despite repeated warnings from the US military.
The Syrian military said that the jet shot down had been carrying out anti-Islamic State operations, and that its lone pilot was killed in the strike.
It warned of "the grave consequences of this flagrant aggression".
According to the Syrian statement, the plane was carrying out operations against Islamic State (IS, formerly ISIS/ISIL) in the countryside around Raqqa when it was targeted, leading to a crash and the loss of the pilot, who is now missing. "What is this if not an act of aggression?"
There is a possibility that as the pockets of resistance in Raqqa diminish, that there could be confrontations between pro-Assad forces and the Arab-Kurdish coalition allied with the US.
Coalition forces stationed at the At Tanf garrison have conducted several airstrikes in the past month in response to incursions into the deconfliction zone on the Iran-Syria border by pro-regime fighters, bombarding militia convoys, technicals and artillery pieces.
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 Raqa.
Iran's focus is increasingly on the border region between Syria and Iraq.
The forces, armed with tanks and small arms, have amassed a few dozen miles outside the base that U.S., British and Norwegian special operations forces use as a staging ground to train, equip and fight alongside their rebel partners.
The Defence Ministry said it was suspending its interaction with America on preventing air incidents over Syria from June 19. The missile launch marks an escalation in Iran's participation and is likely to trigger new terror attacks by ISIS on Iran.
The latest conflict comes as US President Donald Trump hammered Syrian President Bashar al-Assad's murderous regime hard with a salvo of missiles back in April.
"The regime got too close and it got burned".
"The regime is trying to reach the oil province of Deir Ezzor through Raqa", Observatory head Rami Abdel Rahman told AFP.
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