"An intense intervention was made on the group and it was detected that the terrorist group had suffered heavy casualties."
A Turkish military official told Reuters news agency that around 100 special forces were also sent into northern Iraq.
However, a senior leader of the PKK in Iraq, near Turkey's border, denied that the Turkish army had attacked its fighters.
"Our area is quiet. Nothing has happened. There are no air strikes nor any artillery shells," the official told AFP news agency.
In the Iraqi Kurdish city of Arbil, Fuad Hussein, chief of staff for Massud Barzani, president of Iraq's Kurdish region, did not categorically confirm the strikes but said "it could be artillery shelling."
He said a ground assault by Turkish forces was unexpected given the "prevailing weather conditions."
The army operation came a day after Recep Tayyip Erdogan, the Turkish prime minister, said his cabinet had authorised the armed forces to conduct a cross-border operation against Kurdistan Workers Party (PKK) fighters in northern Iraq.