The Kurdistan Workers' Party (PKK) militants launched the attack overnight, near the border town of Cukurca, targeting a military unit which had been stationed there as reinforcement after a significant escalation of rebel violence since June, the sources said.

Six soldiers were killed and several wounded in the ensuing clashes, they said.

There was no immediate word about any PKK casualties.

An operation to pursue the assailants was continuing in the area Tuesday after the army deployed reinforcements, backed by air cover, they added.

It was not immediately clear whether the militants had sneaked in from northern Iraq, where the PKK has long taken refuge at remote mountainous bases.

The PKK dramatically stepped up its 26-year separatist campaign after its jailed leader Abdullah Ocalan said through his lawyers in late May he was abandoning efforts to seek dialogue with Ankara.

The flaring unrest dealt a severe blow on an already fragile government initiative, announced last year, to expand Kurdish freedoms and boost investment in the Kurdish-majority southeast in a bid to erode separatist sentiment in the region and cajole the rebels into laying down arms.

The government rejects dialogue with the PKK, listed as a terrorist group by Ankara and much of the international community, dismissing criticism by Kurdish activists that any peace effort is doomed to fail unless the PKK is included.

Ankara has in recent years granted the Kurds a series of cultural freedoms, but has failed to draw up a clear strategy on how the rebels could be persuaded to abandon violence and reintegrated into society.

The PKK has threatened to spread violence to urban centres in western Turkey.

Last month, radical Kurdish militants claimed responsibility for a remote-control roadside bomb that hit a bus carrying army personel in Istanbul, killing five soldiers and the teenage daughter of an officer.

Under pressure for tougher measures against the rebels, Prime Minister Recep Tayyip Erdogan said last week Ankara was planning to deploy specially-trained professional soldiers along the Iraqi border to fight the rebels and stop them infiltrating Turkish soil.

Since 2007, the Turkish army has often bombed PKK hideouts in northern Iraq and carried out a number of cross-border ground operations to pursue the rebels.

The PKK took up arms for Kurdish self-rule in the southeast in 1984, sparking a conflict that has claimed some 45,000 lives.