Street fighting erupted in the main town of Pakistan's Swat valley on Saturday as security forces mounted a new phase of their offensive against Taliban militants, the military said, Reuters reported.
The battle for control of Mingora is crucial to the success of the offensive launched this month to regain control of the Swat valley and stem a spreading Taliban insurgency.
"Street fighting has begun in Mingora," military spokesman Major General Athar Abbas told a news conference.
He said government forces had cleared parts of the town but fierce clashes were underway in the center of Mingora, and 17 militants had been killed on Saturday.
The operation would be slow as security forces wanted to avoid civilian casualties, he said, although most residents had moved out and only five to 10 percent were still in the town. He feared militants could use civilians as "human shields."
"The pace of the operation will be painfully slow. So be patient but the operation has started and inshallah (God willing) we are going to take it to a logical conclusion."
About 15,000 security forces are taking part in the Swat offensive, which followed intense U.S. pressure on Pakistan to tackle Taliban militants.
Pakistan says more than 1,000 militants and more than 50 soldiers have been killed in the fighting. There has been no independent confirmation of those estimates.
Abbas said around 1,500 to 2,000 "hardcore" militants were believed to be involved in fighting in Swat as a large number of criminals who had joined the Taliban had fled the region.
While Pakistan's government and military have vowed to quell the insurgency in Swat, the country could face greater turmoil in the months ahead.
Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, has said a U.S. military offensive in southern Afghanistan could push Taliban fighters into Pakistan.
The United States is pouring thousands of extra troops into Afghanistan this year to try to reverse gains by the Taliban.
Pakistani military officials say forces have secured several parts of Swat, including two Taliban strongholds, as well as much of the neighboring strategic valley of Buner, around 100 km (60 miles) from the capital Islamabad.
Officials say tribesmen in other districts of the region were raising militias, known as "lashkars," to prevent militants from expanding their influence into their areas.