BRUSSELS (AP) — U.S. Defense Secretary Mark Esper is lashing out at Turkey for its military assault on Syrian Kurdish fighters across the border into Syria.

His remarks in Brussels Thursday came after spending four tumultuous days engulfed in the chaotic ramifications of the withdrawal of U.S. troops from Syria.

Esper says Turkey's unwarranted invasion into Syria jeopardizes security gains made in recent years as the U.S.-led coalition and allied Syrian Kurdish forces battled the Islamic State group. His comments came at the German Marshall Fund.

President Donald Trump announced Wednesday that the U.S. is lifting sanctions on Turkey after the NATO ally agreed to permanently stop fighting Kurdish forces in Syria. Esper was in Iraq Wednesday to discuss the withdrawal and the Islamic State threat with Iraqi leaders and his military commanders.