The K'o-sa was a Uyghur

Uyghur people

The Uyghur are a Turkic ethnic group living in Eastern and Central Asia. Today, Uyghurs live primarily in the Xinjiang Uyghur Autonomous Region in the People's Republic of China...

 tribe mentioned by ancient Chinese texts. D.M. Dunlop believed that they were connected with the Khazars

Khazars

The Khazars were semi-nomadic Turkic people who established one of the largest polities of medieval Eurasia, with the capital of Atil and territory comprising much of modern-day European Russia, western Kazakhstan, eastern Ukraine, Azerbaijan, large portions of the northern Caucasus , parts of...

 and thus postulated an Uyghur rather than Hunnish origin for that people. The K'o-sa, whose belonged to a Tujue tribe, were first mentioned under Du Huan's accounts on Tongdian

Tongdian

The Tongdian is a Chinese institutional history and encyclopedia text. It covers a panoply of topics from high antiquity through the year 756, whereas a quarter of the book focuses on the Tang Dynasty. The book was written by Du You from 766 to 801...

 and possess the areas north of Syria

Syria

Syria , officially the Syrian Arab Republic , is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest....

 and the Byzantine Empire

Byzantine Empire

The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

. They existed under a variant name in Suishu and formed part of the Tiele confederation whose presence was around or close to the Caspian Sea

Caspian Sea

The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...

.