Turkey said on Friday it had sent ground troops into northern Iraq in a significant escalation of its fight against separatist PKK Kurdish rebels, as the US and the European Union called for restraint amid fears the move could undermine Iraq’s only stable region.

An incursion across the border by Turkish troops has long been in the offing, although it was not immediately clear how big the operation was or how long it would last. But it is a serious escalation of Turkey’s battle against the rebels after weeks of aerial assaults on PKK bases in the mountains that straddle the Turkish-Iraqi border.