The sources confirmed reports in the Turkish media this week that a brigadier general in charge of regional operations in Syria had used his official car to smuggle people through checkpoints without being notice, earning thousands of dollars.

It was not clear whether he was ever in the car at the time.

Defence ministry sources confirmed his detention, without saying when or where it took place.

The reports said he was detained in Ankara following an order from the prosecutor in Akcakale district of Sanliurfa province near the Syrian border.

The sources confirmed an investigation had been opened and the officer had been forced to retire shortly afterwards.

"Administrative and judicial processes are under way," the sources said, indicating they would pursue any lawbreakers within the army, regardless of rank.

Since 2016, Turkey has carried out successive ground operations to expel the Kurdish People's Protection Units (YPG) from northern Syria, where its proxies now control two large border strips.

Ankara views the YPG, which dominates the Kurdish-led Syrian Democratic Forces, as an offshoot of the Kurdistan Workers' Party (PKK), which it has outlawed as a "terrorist" group.

The civil war in Syria has killed more than half a million people since it erupted in 2011 after Damascus brutally cracked down on anti-government protests.