Members of the Kurdish security force in northern Iraq reported hearing explosions and gunfire in and around Hakurk, a prominent Kurdistan Workers' Party (PKK) stronghold some 20 kilometres (12 miles) from the Turkish frontier.

More than a dozen Turkish warplanes could be seen heading for the area.

The Firat news agency, considered to be a PKK mouthpiece, said about 5,000 Turkish soldiers and 60 tanks were advancing in the region of Haftanin near the border town of Zaho, west of Hakurk.

Turkish troops crossed into northern Iraq Thursday evening in the largest cross-border offensive in years against PKK hideouts in the region, bombing rebel positions and fighting the militants on the ground.

Ankara says an estimated 4,000 PKK rebels are holed up in northern Iraq and use the region as a springboard for attacks on Turkish territory as part of their campaign for self-rule in Kurdish-majority southeast Turkey.

The conflict has claimed more than 37,000 lives since the PKK, listed as a terrorist group by Ankara and much of the international community, took up arms in 1984.