ISTANBUL (Reuters) - Turkish President Tayyip Erdogan upbraided the United States for its support of Syrian Kurdish PYD rebels on Wednesday, saying Washington's inability to understand the group's true nature had turned the region into a "sea of blood".

"Are you on our side or the side of the terrorist PYD and PKK organization?" Erdogan said in a speech in Ankara to provincial officials, referring to Washington's backing of the Syrian Kurdish fighters PYD against the Islamic State.

Turkey considers the PYD to be a terrorist organization, citing its ties to the Kurdistan Workers Party (PKK), which has carried out a three-decade violent insurgency for Kurdish autonomy in Turkey's mainly Kurdish southeast.

(Reporting by Ayla Jean Yackley and Asli Kandemir; Writing by David Dolan)