ISTANBUL/BEIRUT, Apr 25 (Reuters) Turkish warplanes bombed Kurdish militants in Iraq's Sinjar region and in northeastern Syria today, killing at least 18 fighters and officials in a widening campaign against groups affiliated with the outlawed Kurdistan Workers Party (PKK). The air strikes in Syria targeted the YPG - a key component of the Syrian Democratic Forces (SDF), which are backed by the United States and have been closing in on the Islamic State bastion of Raqqa. It showed the challenges facing the US-led campaign to defeat Islamic State in Syria and risked increased tension between NATO allies Washington and Ankara over Kurdish combatants who have been crucial in driving back the jihadists. At least 18 YPG fighters and media officials were killed in today's air raids, the British-based Syrian Observatory for Human Rights monitoring group reported. There was no immediate casualty report from the YPG. The Turkish military said the two regions it struck had become "terror hubs" and the aim of the bombardment was to prevent the PKK from sending weapons and explosives for attacks inside Turkey. "To destroy these terror hubs which threaten the security, unity and integrity of our country and our nation and as part of our rights based on international law, air strikes have been carried out ... and terrorist targets have been struck with success," the Turkish army said in a statement. The aerial bombardment was carried out around 2 a.m. local time (0430 IST), it added. The YPG said in a statement its headquarters in Mount Karachok near Syria's frontier with Turkey had been hit, including a media centre, a local radio station, communications facilities and military institutions. Turkey has regularly bombed the mountainous border area between Iraq and Turkey where PKK militants are based since a ceasefire broke down in July 2015. But today's raid was the first time they have targeted its affiliate in the Sinjar area, a separate group to the YPG. The PKK established a presence in Sinjar after coming to the aid of its Yazidi population when Islamic State militants overran the area in the summer of 2014 and killed and captured thousands of Yazidis. Turkish President Tayyip Erdogan has said he will not allow Sinjar, around 115 km (70 miles) from the Turkish border, to become a "new Qandil", referring to a PKK stronghold near the borders of Turkey, Iraq and Iran. The presence of a PKK affiliate in Sinjar is also rejected by Kurdish authorities that run their own autonomous region in northern Iraq and enjoy good relations with Turkey. Five members of Iraqi Kurdish peshmerga forces, which are also deployed in Sinjar, were killed, and nine wounded in one of the Turkish air strikes, according to the peshmerga ministry, apparently by accident. It said the attack was "unacceptable" but blamed the PKK for being there and demanded the group withdraw from Sinjar. Designated a terrorist group by Turkey, the United States and the European Union, the PKK has waged a three-decade insurgency against the Turkish state for Kurdish autonomy. More than 40,000 people have been killed in the conflict. REUTERS SDR 1612