Despite joining the war against the Islamic State, Turkey doesn’t want its allies aiding the group’s Kurdish rivals.

Turkey denied American and other NATO aircraft use of its Incirlik base for missions that support Kurdish fighters in Syria, the country’s Foreign Ministry said on Wednesday — raising suspicion that the real motive for its involvement in the Syrian conflict is to suppress Kurdish nationalism.

Late last week, Turkey allowed the United States to fly missions out of Incirlik Air Base against militants of the radical Islamic State militant group in Syria. Its own jets started bombing both Islamic State and Kurdish insurgent targets around the same time.

Turkey previously shied away from fighting the Islamists on its border, fearing that military action against the caliphate would benefit its main rival for control of the north of Syria: Kurdish fighters who are affiliated with the militant Kurdistan Workers’ Party, or PKK.

Turkey’s Islamist government also backed like-minded opposition forces in Syria against the regime of President Bashar al-Assad. It denies ever abetting the Islamic State — a fanatical movement that controls swaths of the desert between Aleppo in northwest Syria, Mosul in northern Iraq and Ramadi to the south, near Baghdad. But Turkey did allow weapons and foreign fighters to cross its border with Syria indiscriminately from 2012 to 2014.

Western countries had long urged Turkey, which has the largest army in the region, to join the campaign against the Islamic State.

Led by the United States, an alliance of Arab and Western countries started an air war against the group last year.

Turkey was prompted into action when an Islamic State supporter blew himself up in Suruç, a mainly Kurdish town close to the Syrian border, last week, killing 32 activists and aid workers. The PKK retaliated by killing four Turkish policemen and injuring several more.

The government also accused the PKK of blowing up a pipeline in eastern Turkey on Monday that carries natural gas from Iran.

At a NATO meeting in Brussels on Tuesday, Turkey’s allies urged restraint but downplayed any connection between their war on the Islamic State and Turkish efforts against the Kurds.

John Kirby, a spokesman for the American State Department, insisted the simultaneous Turkish action against Kurdish militants and the Islamic State in Syria was a coincidence. “The attacks against the PKK were in retaliation for attacks they, the Turks, endured,” he told reporters in Washington DC.

Yet the “safe zone” Turkey has proposed to carve out in northern Syria overlaps with the only segment of the border that the Kurds do not yet control. Situated north of Aleppo, once Syria’s biggest city, it is still in the hands of the Islamic State.

The Turks worry that if the Syrian Kurds would control the whole frontier and link up their territory with that of the Kurdistan Regional Government in Iraq, it might inspire the country’s own sizable Kurdish minority to push for independence as well.

President Recep Tayyip Erdoğan said last month Turkey would “never allow” an independent Kurdistan to be declared on its southern border. “We will continue our fight in that respect whatever the cost may be,” he vowed.