Three Turkish soldiers died Saturday in a mortar attack by Kurdish rebels launched from Iraqi territory, the Turkish defence ministry said.
"Three of our brothers in arms died as martyrs after mortar fire from northern Iraq by terrorists," the ministry said, in reference to the Kurdistan Workers Party (PKK) group.
A fourth soldier was wounded, it said in a statement.
The attack targeted a military base in the border province of Hakkari, the ministry said, adding that the Turkish army had responded with aerial bombardments and artillery fire.
The PKK, branded a terrorist group by Turkey, the United States and the European Union, has waged an insurgency against the Turkish state since 1984. The conflict has claimed more than 40,000 lives.
The PKK's deployment in northern Iraq has been a constant source of tension between Baghdad and Ankara, with Turkey pressing Iraq to play a bigger role in fighting the group.
The defence ministry also said that another Turkish soldier had been killed Saturday in northern Syria by the People's Protection Units (YPG), a Kurdish militia closely allied with the PKK.
Unlike the PKK, however, the YPG has the backing of Western powers in its fight against the Islamic State group.