At least 26 Turkish soldiers have been killed in clashes with PKK militants at police and army posts in south-east Turkey, the government says.
The attacks, in the mainly Kurdish province of Hakkari, are thought to have inflicted the biggest loss on Turkish security forces in years.
In response, Turkish troops are reported to have crossed into northern Iraq where the rebels are based.
President Abdullah Gul has vowed a “great vengeance”.
The attacks come a day after a blast in the south-east Bitlis province killed five police officers and three others.
President Gul recently visited troops in the region to boost morale in an area that has recently seen a spike in violence by PKK militants.