Up to 15,000 Syrian rebels are ready to join a Turkish military offensive against US-backed Kurdish forces in northeast Syria, a spokesman for the main Turkish-backed Syrian rebel group said on Thursday. There is no set date for the operation, which will start from both Syrian and Turkish territory, said the spokesman for the National Army, a Turkish-backed rebel force aimed at unifying disparate factions in northwest Syria. “The battle will be launched simultaneously from several fronts,” Major Youssef Hamoud told Reuters. President Recep Tayyip Erdogan said on Wednesday that Ankara will launch the military operation in a few days, targeting a border region east of the Euphrates river which is held by the YPG Kurdish militia. The Pentagon said any unilateral military action into northeast Syria would be unacceptable. The US has been supporting the YPG in the fight against Islamic State (IS, formerly ISIS) since 2015. Turkey considers the YPG a terrorist organization and an extension of the outlawed Kurdistan Workers Party (PKK).