"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."
At the political level, thousands of people on Saturday turned out to stage an anti-war rally in a Turkish town close to the Iraqi border.
Demonstrators chanted support for the PKK, and police responded by opening fire and arresting more than a dozen people.
Interviewed on Sunday, Cengiz Aktar, a Turkish political analyst, told Al Jazeera: "Apparently these are routine operations. Surgical pinpointed operations that will take place in the course of the next 11 months that the Turkish army was granted by the civilian parliament [to do so].
"That was cleared ... during the meeting between Prime Minister Erdogan and President Bush on November 5.
"We don't expect a big operation, but we might have limited operations.
"This one looks more like a PR [public relations] excercise, because the US military even denied it took place.
"The Americans would hate the idea of seeing another type of chaos in Iraq ... As for today, we don't see any opportunity for the Turkish army for a major and massive land operation."
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 PKK fighters in northern Iraq.