(Bloomberg) -- Turkish President Recep Tayyip Erdogan on Friday pledged to order a military campaign against U.S.-backed Kurdish forces who now control most of Syria’s northeast.

“We will soon crush nests of terrorists to the east of the Euphrates” river, Erdogan told commandos at an army base in Isparta, referring to YPG fighters the government considers to be linked to the Kurdish PKK separatist group it has been fighting for decades.

The offensive would include Manbij after the U.S. failed to deliver on its promise to pull Kurdish fighters from the strategic town, he said.

“They did not withdraw, whatever is necessary will be done,” Erdogan said in a separate speech. Turkey this year seized Syria’s Afrin region from the YPG militia.

To achieve his aims further east, Erdogan would run the risk of a direct confrontation with American forces deployed with the Kurds. The U.S. backed the YPG as the most effective counter to Islamic State after the group rampaged across swaths of Syria and Iraq from 2014.

With the jihadists largely pushed back to remote pockets of territory, the presence of American forces offers Washington some leverage over the Syrian war’s endgame, which is being orchestrated by Russian leader Vladimir Putin.

Erdogan’s vow to march on the YPG in eastern Syria coincided with the trial of an American pastor, Andrew Brunson, whose two-year incarceration on terrorism charges has brought Turkey’s ties with Washington to the brink of collapse.

Even if the Brunson case is resolved, there are outstanding issues with Washington, including its support for “offshoots of the PKK,” in Syria, Ilnur Cevik, a chief adviser to Erdogan, said in an article in the Daily Sabah newspaper on Friday.

To contact the reporter on this story: Selcan Hacaoglu in Ankara at shacaoglu@bloomberg.net

To contact the editors responsible for this story: Benjamin Harvey at bharvey11@bloomberg.net, Mark Williams, Paul Abelsky

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