A top Russian diplomat on Monday condemned the United States for shooting down a Syrian Air Force fighter jet the previous day as an act of “aggression,” while U.S.-backed opposition forces on the ground warned Syrian government troops to stop their attacks or face retaliation.

The U.S. military confirmed a U.S. F-18 Super Hornet shot down a Syrian SU-22 on Sunday, after it dropped bombs near the U.S. partner forces known as the Syrian Democratic Forces. SDF fighters are aligned with the Americans in the campaign against the Islamic State group.

Russia has been a staunch supporter of Syria’s beleaguered President Bashar Assad and has been providing an air cover for his offensive against IS since 2015.

Russian Deputy Foreign Minister Sergei Ryabkov, in comments to Russian news agencies, compared the downing to “helping the terrorists that the U.S. is fighting against.”

“What is this, if not an act of aggression,” he asked.

Meanwhile, the U.S.-backed opposition fighters said Assad’s forces have been attacking their positions in the northern province of Raqqa and warned that if such attacks continue, the fighters will take action.

Clashes between Syrian troops and SDF would escalate tensions in the country and open a new front line in the many complex battlefields of the civil war, now in its seventh year. Clashes between the Kurdish-led SDF and Syrian forces have been rare and some rebel groups have even accused them of coordinating on the battlefield.

The clashes come as both sides are fighting against the Islamic State group, with SDF fighters now focusing on their march into the northern city of Raqqa, the de facto capital of IS.

Government forces have also been attacking IS in northern, central and southern Syria, seizing 25,000 square kilometers (9,600 square miles) and reaching the Iraqi border for the first time in years.

SDF spokesman Talal Sillo said the government aims to thwart the SDF offensive to capture the city of Raqqa. He said government forces began attacking SDF on Saturday, using warplanes, artillery and tanks in areas that SDF had liberated from IS.

Sillo also warned that if “the regime continues in its offensive against our positions in Raqqa province, this will force us to retaliate with force.”

The Britain-based Syrian Observatory for Human Rights, which tracks Syria’s war, said government forced captured from IS on Monday the town of Rasafa, expanding their presence in Raqqa province.

(AP)