The US-backed Syrian Democratic Forces (SDF) has rejected a Turkish army statement that the Islamic State in Iraq and Syria (ISIS) was present in the Afrin region of north-western Syria, where Ankara launched an offensive four days ago, raising worldwide concern.
The United States has around 2,000 special forces troops in Syria, officially as part of an worldwide US-led coalition, assisting the Kurds in battle against Islamic State.
This comes as USA -backed Syrian Democratic Forces reject a claim by Turkey that ISIS is present in the Afrin region.
Erdogan, in a speech to officials in Ankara on Wednesday, reiterated the threat to advance on Manbij as he chided the United States and, in particular, former President Barack Obama, for betraying Turkey by supporting the Syrian Kurdish fighters.
"Russian President Vladimir Putin's spokesman Dmitry Peskov has said that Moscow is "carefully watching the operation" in Afrin" and is in touch with both the Turkish and Syrian governments.
It is understood that on Tuesday he left with two others from London and Leeds to join the YPG and fight in Afrin, Syria, which borders Turkey.
The operation - dubbed "Olive Branch" - was launched on January 20 and focuses on a Kurdish-held area close to the Turkish border.
While a White House statement said Mr Trump had "urged Turkey to de-escalate" its Afrin operation, the Turkish foreign minister later insisted that Mr Erdogan had demanded United States troops withdraw from northern Syria's Manbij region, which is also controlled by Kurdish forces.
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The Pentagon, meanwhile, said Thursday said Turkey's military operations in Afrin are not helpful and threaten to damage the ongoing fight against IS in Syria. They were useful at the time, but it's more important to keep Turkey happy.
"(But) in order for us to discuss the security zone or any other issue with the USA, we have to reestablish trust", Foreign Minister Mevlut Cavusoglu told reporters.
The US and other governments have urged for a de-escalation of tensions, calling on Turkey to "exercise restraint in its military actions" and ensure "its operations are limited in scope and duration".
Just last week, Erdogan promised to "drown" Kurdish forces that the United States is training near Syria's border with Turkey.
Washington had previously proposed a 10-kilometer safe zone. But American commanders know that it's the Kurds (who the Turks now say they want to destroy) who have done the bulk of the fighting and dying. IS announced a new offensive against Syrian government forces on its social media accounts.
The White House statement concluded saying, "The two leaders pledged to improve the strategic partnership between the USA and Turkey, particularly in "fostering regional stability and combating terrorism in all its forms", including ISIS, the Kurdistan Workers' Party (PKK), al-Qa'ida, and Iranian-sponsored terrorism".
The U.S. leader also voiced concern, as tensions spike between the North Atlantic Treaty Organisation allies, about "destructive and false rhetoric coming from Turkey", the White House said.
Ankara has been infuriated by US support for the YPG, which it sees as a domestic security threat, one of several issues that have brought relations between the United States and its Muslim NATO ally close to breaking point.