At least 18 Kurdish fighters battling the so-called Islamic State in Syria have been killed in air strikes by Turkey. The Popular Protection Units (YPG) militia said its positions were hit by multiple strikes early on Tuesday. The group is backed by the United States, which sees it as a key component of the fight against IS. But Turkey said it hit "terror hubs" linked to the banned Kurdistan Workers Party (PKK). The Syrian Observatory for Human Rights, a UK-based monitoring organisation, said a dawn strike on Tuesday morning targeted Kurdish positions in Hassakeh province in north-east Syria, hitting a media centre and radio station. A separate series of strikes hit a base near Sinjar in northern Iraq, close to the Syrian border, killing at least six fighters from the Peshmerga, the military forces of the autonomous region of Iraqi Kurdistan. In a statement, the Peshmerga - which has friendly relations with Turkey - said that while the attack was "unacceptable", it blamed PKK forces in the area, and called on them to withdraw. A US military officer accompanied YPG commanders on a tour of the sites in Syria later on Tuesday to demonstrate the close partnership, Reuters news agency said. Turkey views the YPG as an extension of the PKK, a Turkish-Kurdish rebel group that has been fighting an armed struggle against the Turkish government since the 1980s. A ceasefire between Turkey and the PKK ended last year and clashes have since claimed hundreds of lives on both sides. But the YPG also form the backbone of the US-backed Syrian Democratic Forces (SDF) - a force allying Kurdish and Arab fighters battling IS. In a statement, the Turkish military said "the determined terrorist targets were hit" and that it had struck because of weapons being carried into Turkey. "The operations will continue with the same determination until the last terrorist has been neutralised," it said. A commander for the Kurdish forces in Syria called on its international allies to defend their forces from further attacks. "We are asking the international coalition to intervene to stop these Turkish violations," the unnamed commander told the AFP news agency. "It's unthinkable that we are fighting on a front as important as Raqqa while Turkish planes bomb us in the back," he said, referring to the IS stronghold in northern Syria. The Kurdish-led Syrian Democratic Council (SDC), a political wing for the SDF, said Turkey's offensive was aiding IS.