WASHINGTON (Reuters) - A U.S.-led coalition sent aircraft into northeastern Syria on Thursday in a “very unusual” move to protect American special operation ground forces from attacks by Syrian government jets, a Pentagon official said on Friday.

Pentagon spokesman Captain Jeff Davis told reporters the coalition aircraft reached the area around the city of Hasaka as the two Syrian SU-24s were leaving, and the U.S. special operation forces were in the area where the strikes were taking place. He said the Syrian planes did not respond to efforts by ground forces to contact them.

Davis said he was not aware of any other instances where coalition aircraft had been scrambled to respond to Syrian government bombing.

“This is very unusual, we have not seen the regime take this kind of action against YPG before,” Davis said, using an acronym for the Syrian Kurdish fighters.

Additional combat air patrols have been sent to the area in order to protect the ground forces.

On Friday, two Syrian aircraft tried to pass through the airspace around Hasaka, but left without incident when they were met by coalition fighter jets. The coalition fighter jets were F-22 aircraft and came within 1 mile (1.6 km)of the Syrian planes.

On Friday, the Syrian government carried out a second day of airstrikes and artillery bombardment, causing thousands of civilians to evacuate Kurdish areas of the city. Dozens of civilians have been killed over the last 48 hours.

The Russians were contacted through a channel used for air safety, and they made clear the bombing was not being carried out by their jets, Davis said.

The Russians were asked to tell the Syrian government that U.S. aircraft would defend its troops on the ground if they were threatened, Davis said.

“The Syrian regime would be well advised not to interfere with coalition forces or our partners,” he said, adding that the United States had the right to defend its troops.

Faysal Itani, a senior fellow at the Atlantic Council think tank, said it was easy for the Syrian government to avoid bombing large bases, but far more risky to strike areas close to YPG forces being trained by U.S. forces.

“If the regime continues bombing in close proximity to U.S. forces, then simply I think the United States will shoot down the regime aircraft,” Itani said.