At least 12 people were killed after Kurdish separatists ordered them off a bus in south-east Turkey and opened fire, Turkish officials have said.
The attack follows the recent killing of a senior PKK figure
Civilians and government-backed guards were said to be among the victims of the ambush, which happened on Saturday.
The attack, in Sirnak province near the Iraqi border, followed the killing of a Kurdistan Workers Party (PKK) leader.
The Turkish Prime Minister, Recep Tayyip Erdogan, condemned the ambush, saying it would not go unpunished.
"Our determination to fight the terrorists will continue," he said. "They will definitely get a response."
Mr Erdogan said a resurgence in attacks was a sign of the PKK's "distress" at an army crackdown on its members.
On Sunday, two Kurdish rebels were killed in an incident on the Iraqi border, local media said.
The Turkish government in Ankara says 4,000 PKK fighters are operating from the Iraqi side of the border.
Turkey has been considering crossing the border to pursue the group.
But on Friday Iraq denied Turkey permission to pursue armed separatists onto Iraqi territory.
Instead Iraq and Turkey signed a wide-ranging security agreement, pledging to prevent finance, logistical support and propaganda for the PKK.
Ankara has warned Baghdad to crack down on Kurdish rebels in Iraq or face a possible incursion by Turkish troops.
The PKK has been fighting for autonomy in south-eastern Turkey since 1984 and more than 30,000 people have been killed in the conflict.
The PKK has been labelled a terrorist organisation by Turkey, the US and the EU.