The incident drew sharp criticism from the Pentagon. Rankine-Galloway said that U.S. aircraft would defend coalition forces on the ground and that Syrian forces “would be well advised not to interfere” with operations in the area.

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U.S. aircraft arrived in the area as the Syrian aircraft were leaving. Rankine-Galloway said U.S. forces contacted their Russian counterparts through pre-established channels that ensure the two countries avoid any potential incidents in the region. When the Russians indicated that the planes bombing near the coalition forces were not theirs, the U.S. launched a “combat air patrol,” Rankine-Galloway said. While he would not specify from where the U.S. aircraft launched, the United States maintains a contingent of F-15 fighter jets specifically designed for air-to-air combat at Incirlik Air Base in Turkey.

During the bombing, local forces on the ground attempted to contact the Syrian aircraft through an internationally recognized radio channel used to hail aircraft called a “guard” frequency, but got no response.

The U.S. has since stepped up air patrols in the area to ensure coalition forces remain protected, officials said.

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According to a senior defense official, who spoke on the condition of anonymity to discuss a developing situation, two Syrian Su-24s returned to the airspace near Hasakah on Friday but were deterred from entering the area by coalition aircraft. No weapons were fired by either side, the official said.

Tucked in the northeastern corner of Syria, Hasakah is seen primarily as Kurdish territory and has remained mostly quiet since major fighting ended there last year, making the strikes on the city all the more unusual. In the summer of 2015, Islamic State forces attempted to wrest the city from Kurdish and Syrian military forces, but were eventually repelled. Since then, there have been bouts of fighting between pro-Syrian government defense forces and Kurdish militias, though Syrian warplanes had avoided the city until recently. The contingent of local fighters stationed in and around Hasakah are primarily made up of the Syrian Democratic Forces and Kurdish Peoples Protection Units, known as the YPG.

U.S., British, and French Special Operation troops have been advising local forces fighting in northern and eastern Syria since late last year.

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Last week, the SDF, backed by U.S. airstrikes and Special Operations forces wrested the Syrian city of Manbij from the Islamic State. Manbij, once a key hub for foreign fighters and equipment bound for the militant group, is roughly 170 miles west of Hasakah and took the U.S.-backed forces nearly four months to retake.

This post has been updated.