Turkish ground forces have crossed the border into northern Iraq to target Kurdish rebels said to be sheltering there, Ankara has said.
It said the raid began late on Thursday after an air and artillery bombardment.
Prime Minister Tayyip Erdogan has said the offensive is limited in scale and troops will return as soon as possible.
Iraqi officials say they remain unaware of any major incursion into the north, and Kurdish officials believe it took place in a remote and unpopulated area.
"The target, purpose, size and parameters of this operation are limited," said Mr Erdogan, whose comments were televised.
"Our armed forces will come back in the shortest time possible as soon as they achieve their objectives," he said, insisting that members of the Kurdistan Workers' Party (PKK) were the sole targets.
Correspondents say the aim is to isolate the organisation and prevent it using northern Iraq as a launch pad for attacks on Turkish soil.
