There are frequent clashes with between troops and rebels
Kurdish rebels have killed 15 Turkish soldiers near Semdinli in the country's south-east, the military says.
The soldiers died during an attack by fighters said to be from the Kurdistan Workers' Party (PKK), based over the border in northern Iraq.
Generals said troops fought back, killing 23 rebels, but that two soldiers were missing after the attack.
Turkey blames the PKK for a series of bomb attacks on its cities, and often targets rebels with air strikes.
More than 40,000 people are thought to have been killed since 1984, when the PKK launched its campaign for a Kurdish homeland in south-eastern Turkey.
The latest attack was a surprise strike on a military outpost by guerrillas using heavy weapons.
The ambush prompted the most intense fighting and led to the highest number of casualties in a year, says the BBC's Turkey correspondent Sarah Rainsford.
Turkey has launched a number of military incursions across the border into northern Iraq since the start of 2008.
Generals accuse the PKK of using the region as a sanctuary from where to plan and carry out deadly attacks against Turkish troops in the south and eastern regions of Turkey.
Troops and rebels have regularly exchanged fire in the border regions in recent months.
In May, the military said a week of air strikes had killed 150 PKK guerrillas.
In the fiercest fighting, thousands of Turkish troops swept across the border into Iraq in February, before pulling out amid international pressure.
The government, led by Prime Minister Recep Tayyip Erdogan, is currently asking parliament to extend the military's mandate to carry out cross-border operations, something that is now highly likely to be approved, our correspondent says.
Ankara has linked the PKK to a series of high-profile attacks on Turkish targets, notably a series of blasts in Istanbul in July.
Seventeen people died and some 150 others were injured when two bombs exploded in a residential area of the country's largest city.
Also in July three German climbers were taken hostage by the PKK in eastern Turkey. They were later freed unharmed.