Assad has defeated rebels in many parts of western Syria with critical help from Russian Federation and Iran.
In a rare experience U.S. diplomats go through in any host country, their Charge d;Affairs was summoned by Turkish foreign ministry in Ankara to express "discomfort" over Washington's support for Syrian Kurdish fighters on Wednesday.
The Turkish government said the Idlib offensive was endangering the effort to reach a resolution of the conflict and accused the Syrian government of using the pretext of fighting militants to wage a wide scale attack.
On January 9, the Russian and Iranian ambassadors were reportedly called to the Turkish Foreign Ministry in Ankara to receive a protest note.
Earlier on Wednesday, Turkish Foreign Minister Mevlut Cavusoglu had warned Russian Federation and Iran they should adhere to the joint accord they signed with Turkey a year ago in which the three countries announced a "de-escalation zone" in Idlib. Erdoğan has cast the YPG as "terrorists" closely connected to the Kurdish Worker's Party (PKK), which has been fighting an anti-government guerrilla campaign in southeastern Turkey for decades.
The People's Protection Units (YPG), the armed wing of the Syrian Kurdish Democratic Union Party, or PYD, is seen by the United States as the most effective ground partner for the US-led coalition fighting ISIL in Syria.
On Tuesday, Turkey gathered Russian Federation and Iran's ambassadors to air its displeasure, saying the offensive must be called off immediately.
Earlier, Cavusoglu said the country doesn't rule out a new military operation against YPG and PYD in Afrin. This is also the reason for Vladimir Putin's telephone conversation with Recep Tayyip Erdogan.
The three countries had agreed past year to establish a "de-escalation zone" in the opposition-held Idlib province and surrounding region, which borders Turkey. "They will pay the price for it, if need be", reported Anadolu.
Rebels launched a counter attack against Syrian government forces and their allies in Idlib province on Thursday, seeking to roll back an advance that is fueling tension with neighboring Turkey. The Russian defense ministry implied such technologies ware given to the rebels by Western (US) intelligence services and has rejected denials from the Pentagon to the contrary.
Turkey-US relations strained over the latter's support for the PKK/PYD terror group's presence and arming of the group in the region.