The Turkish army said on Thursday a military helicopter that crashed a week ago, during clashes that killed eight soldiers and two pilots, may have been brought down by Kurdish militants with a ground-to-air missile.

If confirmed, it would be the first known usage in recent years of such weaponry by Kurdistan Workers Party (PKK) militants, who have been waging an insurgency for Kurdish autonomy in Turkey’s southeast for more than three decades.

“As the helicopters carried out their mission, the conclusion has been reached that one helicopter may have been struck and downed with an air defense weapon that could have been a missile, possibly fired from the ground,” the military said in a statement.

It said a detailed probe was continuing.

The armed forces initially said the helicopter had crashed last Friday due to a technical fault during air operations against PKK militants in the mainly Kurdish province of Hakkari near the border with Iraq.

After the collapse of a ceasefire between the two camps last July, Turkey’s southeast has seen some of its worst fighting since the height of the Kurdish insurgency in the 1990s.

President Recep Tayyip Erdogan, who had spearheaded the peace process between the state and the PKK, has ruled out any return to negotiations and has vowed to crush the group.

Thousands of people, including hundreds of civilians and 450 Turkish security forces, have been killed in the violence since July.

The violence has also devastated mainly Kurdish districts.

Ankara and its Western allies consider the PKK a terrorist organization.

Erdogan’s office said in a statement earlier that U.S. President Barack Obama had discussed with the Turkish leader in a phone call late on Wednesday strengthening cooperation in fighting all terrorist organizations including the PKK.