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.

Turkish helicopters air-dropped troops in the region, they said, and more than a dozen Turkish warplanes were seen heading for the area.

Suspected rebels positions at four locations across the Turkish border town of Cukurca came under intensive artillery fire, the sources said.

The Firat news agency, considered to be a PKK mouthpiece, reported air raids and fighting in the Zap area to the west and said about 5,000 Turkish soldiers and 60 tanks were advancing towards nearby Haftanin, close to the Iraqi border town of Zaho.

The Turkish army released pictures of the offensive, showing soldiers in white camouflage cloaks aiming at unseen targets in snow-covered mountains and a helicopter landing on a hill.

Turkish forces 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.