Up to 15,000 Syrian rebels are ready to join a Turkish military offensive against US-backed Kurdish forces in northeast Syria, but no date has been set for the operation, a spokesman for the main Turkish-backed Syrian rebel group said on Thursday. President Tayyip Erdogan said on Wednesday that Turkey would soon begin a mission targeting the Kurdish Peoples Protection Units (YPG), which Ankara considers a "terrorist" group. The announcement prompted a sharp rebuke from the Pentagon, which said any unilateral military action into northeast Syria would be "unacceptable". "We will start an operation to free the east of the Euphrates from the separatist terrorist organization in the next few days," Erdogan said during a speech in Ankara, referring to territory held by the Kurdish Peoples Protection Units (YPG). American forces have worked closely with the YPG under the Kurdish-led Syrian Democratic Forces (SDF) alliance, which has played a key role in the war against the ISIS extremist group The Pentagon has repeatedly warned that any fighting between the Turks and the SDF is a dangerous distraction from the core US mission in Syria of fighting ISIS. Pentagon spokesman Commander Sean Robertson said any unilateral military action in northeast Syria would be a "grave concern", as it could potentially jeopardize US troops working with the SDF in the region. "We would find any such actions unacceptable," he said in a statement. Turkey says the YPG is a "terrorist offshoot" of the outlawed Kurdistan Workers Party (PKK), which has waged an insurgency against the Turkish state since 1984. PKK is blacklisted as a terror group by Ankara and its Western allies. "The target is never American soldiers but terrorist organization members active in the region," Erdogan told the audience at a defense industry summit. American forces are with the SDF east of the Euphrates as well as in the flashpoint city of Manbij, which is west of the river. "We should not and cannot allow (ISIS) to breathe at this critical point or we will jeopardize the significant gains we have made alongside our coalition partners and risk allowing (ISIS) to resurge," Robertson said. The YPG also said a Turkish offensive would be to the benefit of ISIS. "The (Turkish) threats coincide with the advance of our forces against the terrorists, this time with the entrance into the town of Hajin," YPG spokesman Nuri Mahmud said. The SDF launched an offensive on September 10 to expel ISIS from the Hajin pocket, on the eastern bank of the Euphrates River close to the Iraqi border. "Any attack on the north of Syria will have a direct impact on the battle of Hajin. The forces who are fighting (there) will return to defend their areas and their families," Mahmud said. Washingtons relationship with the YPG, seen as a key ally, is one of the main sources of tensions between the United States and NATO member Turkey. Ankara has repeatedly lambasted Washington for providing military support to the Kurdish militia. Erdogan has previously threatened to attack areas held by the YPG. In a bid to avoid any clash, the NATO allies agreed a "roadmap" for Manbij in June. Erdogans comments came a day after the Pentagon announced the setting up of US observation posts on the northeast Syria border region intended to prevent altercations between the Turkish army and the YPG despite calls from Ankara not to go ahead with the move. Erdogan claimed Turkey was not being protected from terrorists but "terrorists were being protected" from possible action by Turkey. Turkey has previously launched two operations in northern Syria. The first offensive began in August 2016 with Turkish forces supporting Syrian opposition fighters against ISIS and was completed by March 2017. Then in January 2018, Turkish military forces backed Syrian rebels to clear the YPG from its northwestern enclave of Afrin. In March, the operation was completed with the capture of Afrin city.