Turkey's foreign minister has claimed that American forces in Syria are intentionally stalling the fight against Islamic State militants as an excuse to maintain ties with Syrian Kurdish militiamen, against Ankara's wishes.
Mevlut Cavusoglu said US forces are leaving "pockets" with IS militants intact to justify continued co-operation with the Kurdish militia.
Speaking ahead of a visit by US Secretary of State Rex Tillerson later this week, Mr Cavusoglu said Turkey's ties with the US are at a make-or-break stage, and that Washington needs to take "concrete steps" to regain Turkey's trust.
"Our relations are at a very critical stage," Mr Cavusoglu said. "Either we will improve ties or these ties will totally break down."
Ankara is riled over Washington's support for the Syrian Kurdish People's Protection Units (YPG) - the top US ally in the fight against IS.
The government considers the YPG a terrorist group linked to Kurdish insurgents fighting within Turkey's own borders.
Turkey's military launched a cross-border operation into the Syrian Kurdish-held enclave of Afrin in northern Syria to rout the YPG from the region.
Ankara has also threatened to expand its offensive to the YPG-held town of Manbij, east of Afrin, where the US has a military presence, setting the scene for a potential showdown between the two Nato allies who back different sides in Syria's complex and multi-layered civil war.