ISTANBUL (AP) — Turkey's defense ministry says four Turkish soldiers have been killed in northeastern Syria.

The ministry said the soldiers were killed in a car bombing attack Wednesday while they were conducting road checks. The Turkish military death toll stands at 20 since October, when Turkey launched an offensive against Syrian Kurdish militants.

Turkey considers the Syrian Kurdish People's Protection Units, or YPG, terrorists for their links to a decades-long Kurdish insurgency inside Turkey that has killed tens of thousands. The same fighters were the United States' partners on the ground in fighting the Islamic State group.

Syrian Kurds established an autonomous zone in war-torn Syria's northeast in 2012. Ankara has repeatedly said it would not allow a “terror corridor” along its borders and conducted two other cross-border military operations in northern Syria to break apart YPG-held territory. On all three offensives, the Turkish military has worked with Syrian opposition fighters, who have been accused of atrocities.

Ankara's campaign in October — the so-called Olive Branch operation — drew widespread international criticism and the U.S. was seen as abandoning the Kurdish force that had helped fight IS. Two ceasefires, brokered by the U.S. and Russia, required the Kurdish force to withdraw away from the Turkish border for Turkey to halt its offensive. Joint Russian and Turkish patrols were established.

While the Turkish defense ministry did not assign blame for Wednesday's car bomb attack, it has previously accused the Kurdish militants of similar attacks.