Turkish troops entered northern Iraq overnight, penetrating 10 kilometres (six miles), after deadly attacks by Kurdish rebels inside Turkey, a security official said on Sunday.
Three people were killed in the incursion into the Qandil mountains, where the rebel Kurdistan Workers Party (PKK) maintains rear bases for its 26-year-old armed campaign for self-rule in southern Turkey, he said.
The official did not specify whether the dead were civilians or PKK fighters, but he said that the incursion happened in the Shamarsha district of Arbil province north of the town of Sidikan.
It was the second time in five days that Turkish ground forces had crossed the border. On Wednesday, Turkish troops crossed from Sirnak province into Dohuk province farther west, in their first ground operation across the border in two years.
"Two of our men were killed in the clashes that took place on Wednesday," PKK spokesman Ahmed Denis told AFP in the Iraqi Kurdistan regional capital of Arbil on Friday.
The intensifying clashes between the PKK and Turkish troops prompted Denis to warn on Saturday that the rebels would take their armed campaign to cities across Turkey if the army pressed on with a policy of military confrontation.
Saturday was the bloodiest day in two years for the Turkish army after Kurdish rebels killed 11 soldiers in the far southeast of the country near the border, the army said. According to the Turkish military, 12 PKK fighters died in a counter-attack.
Turkish warplanes also launched bombing raids on suspected rebel targets inside Iraq, both sides said.
The conflict with the PKK, considered a terrorist organisation by much of the international community as well as by Ankara, has claimed more than 45,000 lives since it broke out in 1984, according to the Turkish army.