The US-backed coalition of Kurds and Arabs of the Syrian Democratic Forces (SDF) have begun their long-awaited assault on the Islamic State’s self-proclaimed caliphate in Raqqa. The forecast for the battle has it that it will be long and costly as the IS fighters are cut off from any support.

But the government of Turkey released an ominous warning that they’ll be watching the SDF for any aggression to their own interests. Turkey considers the Kurds terrorists in their country which makes the entire alliance against the IS on shaky ground.

On Tuesday, Prime Minister Binali Yildrim said Turkey would act immediately should any perceived threats arise from the Syrian Democratic Forces’ operations to recapture the city of Raqqa from the so-called “Islamic State” (IS).

Yildrim made his remarks in Ankara to deputies from President Recep Tayyip Erdogan’s ruling Law and Justice Party on the same day that the SDF launched an attack on the IS stronghold from the east, north and west.

Yildrim said Turkey was taking precautions in case its interests were threatened

The SDF, a roughly 50,000-member alliance backed by the United States, largely consists of Arabs and fighters from the People’s Protection Units (YPG), a Kurdish militia formed in 2011 that Turkish officials consider a terrorist organization. The Turkish government has been facing a Kurdish insurgency, primarily from the Kurdistan Worker’s Party (PKK), in areas of southeast Turkey since 1984. Officials in Ankara have expressed concern that a YPG takeover of Raqqa, about 55 miles (90 kilometers) south of Turkey’s border, could embolden Kurds in their fight for autonomy.

But the United States views the SDF as key partners in the fight against IS in Syria and has provided the alliance with air support, training and weapons.

The United States also began arming the YPG directly to help them battle the approximately 3,000-4,000 IS fighters believed to still be in Raqqa.