Turkish security forces have killed 54 Kurdish militants during a major operation to root out Kurdistan Workers Party (PKK) rebels from two urban centres, official media said on Friday.

"Fifty-four terrorists have been neutralised" in in the southeastern towns of Cizre and Silopi, the state-run Anatolia news agency said, using the customary official euphemism for the killing of Kurdish militants.

The operation began on Tuesday, with 25 militants killed in first two days. But the latest figure more than doubled the number of casualties, implying a heavy toll during Thursday's fighting inside the towns.

The vast operation, which is now in its fourth day and reportedly involves some 10,000 troops, is centred on two towns -- Cizre and Silope, both of which are in the Kurdish-dominated Sirnak province where the military has imposed blanket curfews.

The government says the operation is needed to eliminate militants who were effectively taking over the towns by building barricades and digging trenches.

But Kurdish activists have accused the army of acting with impunity and turning large parts of the towns to rubble.

Images from the area show troops clashing with militants in the otherwise deserted streets. Cizre has a population of some 100,000, and Silopi more than 80,000.

The operations mark a new escalation in five months of fighting with the PKK since a two-and-a-half year truce collapsed in July.

The PKK launched a formal insurgency against the Turkish state in 1984, initially fighting for Kurdish independence although it now presses more for greater autonomy and rights for the country's largest ethnic minority. The conflict has left tens of thousands dead.