(Reuters) – American-led forces have sharply intensified air strikes in the past two days against Islamic State fighters threatening Kurds on Syria’s Turkish border after the jihadists’ advance began to destabilize Turkey.

War on the militants in Syria is threatening to unravel a delicate peace in neighboring Turkey where Kurds are furious with Ankara over its refusal to help protect their kin in Syria.

The plight of the Syrian Kurds in Kobani provoked riots among Turkey’s 15 million Kurds last week in which at least 35 people were killed.Turkish warplanes were reported to have attacked Kurdish rebel targets in southeast Turkey after the army said it had been attacked by the banned PKK Kurdish militant group, risking reigniting a three-decade conflict that killed 40,000 people before a ceasefire was declared two years ago.

“For the first time in nearly two years, an air operation was carried out against our forces by the occupying Turkish Republic army,” the PKK said. “These attacks against two guerrilla bases at Daglica violated the ceasefire,” the PKK said, referring to an area near the border with Iraq.

U.S. officials have expressed frustration at Turkey’s refusal to help them fight against Islamic State. Washington has said Turkey has agreed to let it strike from Turkish air base. Ankara has said that is still under discussion.

NATO-member Turkey has refused to join the coalition unless it also confronts Syrian President Bashar al-Assad, a demand that Washington, which flies its air missions over Syria without objection from Assad, has so far rejected.

The unrest shows the difficulty Turkey has had in designing a Syria policy. Turkey has already taken in 1.2 million refugees from Syria’s three-year civil war, including 200,000 Kurds who fled the area around Kobani in recent weeks