Turkey's President Tayyip Erdogan attends the official opening ceremony of Istanbul's new airport, in Istanbul, Turkey, October 29, 2018. REUTERS/Murad Sezer

ISTANBUL (Reuters) - Turkish President Tayyip Erdogan vowed on Tuesday to crush Kurdish fighters east of the Euphrates river in Syria, announcing a major military operation against U.S. allies in an area where Washington supports them with troops on the ground.

Syria’s YPG Kurdish militia are the core of a force that has fought against Islamic State with the support of U.S. air power, arms, funds, training and an estimated 2,000 American special forces troops on the ground.

Turkey, Washington’s main Muslim ally within NATO, considers the YPG an enemy and has already intervened to sweep the fighters from territory west of the Euphrates in military campaigns over the past two years.

Previous campaigns halted at the banks of the river, in part to avoid direct confrontation with Washington. But Erdogan said Turkey was now prepared to press on.

“We will destroy the terror structure east of the Euphrates River. We have completed preparations and plans regarding this issue,” Erdogan said in a speech to lawmakers from his AK Party.

“We have started active intervention operations against the terror organization in the last couple of days. We will soon come down hard on the terror organization with more extensive and effective operations.”

State-owned Anadolu news agency said on Sunday Turkish forces had already bombarded positions east of the river held by the YPG.

Turkey has been infuriated by U.S. support for the YPG, which it considers a terrorist organization and an extension of the outlawed Kurdistan Workers’ Party (PKK) which has fought an insurgency in southeast Turkey for more than three decades.

Erdogan issued what he said was a “final warning” last week to those who would endanger Turkey’s borders. He said then that Turkey would focus its attention on the east of the Euphrates, rather than the Manbij area just west of the river, where U.S. and Turkish forces agreed in June to carry out joint patrols.