An intelligence official said the ground troops were facing 'stiff resistance' from Taliban.

'Ten helicopter gunships and two F-16 jet fighters have repeatedly targeted militant positions since Tuesday to support our forces that are making an advance towards the militants' stronghold,' said the official who spoke on condition of anonymity.

'There are reports of heavy casualties among the Taliban but we do not have the exact figure so far,' he added.

Pakistan has deployed thousands of troops in South Waziristan since mid-June when the government announced it had ordered an operation against Mehsud, who was believed to be behind dozens of suicide attacks across the country.

The onslaught was delayed after assessments that more preparations were required to defeat an estimated 15,000 well-armed and equipped guerrilla fighters from Mehsud's group.

The US has asked Pakistan for an all-out offensive in South Waziristan, from where Islamic insurgents conduct cross-border raids on Western troops in Afghanistan.