Turkish President Tayyip Erdogan addresses his supporters in Usak, Turkey January 20, 2018. Kayhan Ozer/Presidential Palace/Handout via REUTERS

ISTANBUL (Reuters) - President Tayyip Erdogan on Friday promised to sweep militants from the length of Turkey’s Syrian frontier, saying that Ankara could extend its current military operation in northwest Syria all the way east to the border with Iraq.

Turkey’s offensive in Syria’s Afrin against the Kurdish YPG militia, which it views as a security threat, has opened a new front in the multi-sided Syrian civil war and further strained ties with NATO ally Washington.

Since the start of the incursion, dubbed “Operation Olive Branch” by Ankara, Erdogan has said Turkish forces would continue east to Syria’s Manbij, potentially bringing it in confrontation with the U.S. soldiers positioned there.

“Operation Olive Branch will continue until it reaches its goals. We will rid Manbij of terrorists, as it was promised to us, and our battles will continue until no terrorist is left until our border with Iraq,” Erdogan said.

Any push by Turkish forces toward Manbij, part of a Kurdish-held territory some 100 km (60 miles) east of Afrin, could threaten U.S. efforts in northeast Syria. On Thursday, Turkey urged the United States to halt its support for YPG fighters or risk confronting Turkish forces on the ground.

Ankara considers the YPG to be an extension of the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade-long insurgency in Turkey. Washington sees the YPG as an effective partner in the fight against Islamic State in Syria, a stance that has infuriated Turkey.

Turkey has said the United States had proposed a 30 km (19 mile) “safe zone” along its border with Syria, but Foreign Minister Mevlut Cavusoglu said trust between two countries needed to be reestablished first.

“If we will wage a battle against terror together (with the United States), we will either do this together or we will take care of ourselves,” Erdogan said.