Local officials said that an ambush took place near the Silvan town in Diyarbakir province. At least 11 Kurd rebels were also killed in the clash, the officials added.
The army had been conducting an operation in the area, according to local security sources. Fighting has been so fierce it reportedly started a forest fire.
The Turkish prime minister travelled to Ankara for an emergency meeting with defence chiefs.
The attack came hours after the government and a pro-Kurdish party failed to agree an end to a boycott of parliament by Kurdish MPs, who are protesting against court decisions that refused to release five newly-elected pro-Kurdish politicians from jail.
Tensions have been rising since the PKK ended a six-month ceasefire in February.