Pakistani forces fought fierce battles with Taliban militants on Sunday, a day after launching a long-awaited offensive aimed at bringing the writ of state to lawless tribal lands on the Afghan border, Reuters reported.

The offensive on the global Islamist hub of South Waziristan follows a string of brazen militant attacks in different parts of the country, including an assault on army headquarters, in which more than 150 people were killed.

About 28,000 soldiers are battling an estimated 10,000 hard-core Taliban, including about 1,000 tough Uzbek fighters and some Arab al Qaeda members, after surrounding militant territory and pushing in from three direction.

Heavy clashes erupted on Saturday as soldiers backed by aircraft and artillery encountered resistance, and four soldiers were killed and 12 wounded, the military said. There was no information about militant casualties.

Security forces captured a Taliban stronghold at Spinkai Raghzai on Saturday after the militants withdrew from their fortifications and took refuge in nearby mountains, officials said.

"It is a flat area so whenever they tried to put up resistance, the helicopter gunships fired at them so they decided to flee to the mountains," said a senior government official in the northwest. In a show of unity before the offensive, government and political party leaders gave the military full backing on Friday, vowing to weed out militants.

Nuclear-armed Pakistan has been under U.S. pressure to crack down on Islamist militancy as President Barack Obama considers a boost in troop numbers fighting in neighboring Afghanistan.

Many al Qaeda and Taliban members fled to northwest Pakistan after U.S.-led troops ousted the Taliban in Kabul in 2001 and the region has become a global hub for Islamist militancy.

The offensive could be the army's toughest test since the militants turned on the state and it will be hoping Afghan Taliban factions elsewhere in South Waziristan and in North Waziristan stay out of the fight.

Up to 100,000 civilians have fled from South Waziristan in anticipation of the offensive, the army said, while the United Nations said 500 people were leaving every day.

Security force are on alert across the country in case of retaliatory strikes.