An Associated Press reporter saw four armed Taliban on the edge of town, a little more than a mile from an army checkpoint. The army and witnesses have said the militants have dug trenches and laid mines to repel an assault.
Pakistan began operations in the valley and surrounding districts last month following intense U.S. pressure for action against extremists eroding the stability of the nuclear-armed state and attacking American troops in neighboring Afghanistan.
The offensive is shaping up as a major test of the will and ability of the often-criticized army to defeat the militants. The extremists fought the military to a stalemate in Swat last year, forcing the country's embattled politicians to accept a peace deal.
In other regions near the Afghan border, the typically outnumbered insurgents have avoided such conventional battles with the military and melted away into the mountains only to reappear several months later - an outcome that some analysts say is possible this time as well.
The operation was launched after the militants pushed out from Swat to seize a district just 60 miles from the capital, Islamabad, under cover of the since-collapsed peace process. The advance alarmed Washington and many Pakistanis who had previously believed they could negotiate with the militants.
The military said it has killed more than 800 of the estimated 4,000 militants in the region, but the fighting has triggered an exodus of at least 900,000 people, creating a humanitarian crisis that risks undercutting public support for the offensive.
"Wherever the terrorists are present, they need to be eliminated completely," said Mian Iftikhar Hussain, information minister of Northwest Frontier Province. Until last month, he was the leading advocate of moves to make peace with the insurgents.
The army said that in the last 24 hours, it has "achieved success in various areas of Swat" and to have killed 55 more fighters, with the loss of three soldiers. Like much of the information provided by authorities, there was no way to verify those tolls independently.
The army has not given any figures on civilian causalities, but refugees from the region have reported several deaths. Fleeing resident Ismail Khan said he had seen bodies lying in some of the streets of the fast-emptying town but didn't know if they were militants, civilians or soldiers.
The army lifted its curfew in Mingora for eight hours and urged the remaining residents to flee "so that security forces can take the militants to task in street-to-street fighting," army spokesman Maj. Gen. Athar Abbas said.
Columns of cars, trucks and horse-drawn carts packed with people and bundles of possessions streamed out of the town. Some picked their way past bombed-out government buildings and burned-out civilian vehicles along the crowded and cratered main highway.
Others took dirt roads through the fields and mountains.
Many went south on foot with only the clothes on their backs.
"We do not know where we are going," said Muhammad Ismail Khan, who had been unable to find a ride for himself and nine relatives. "We do not know if we will ever be able to come back."
Of the 900,000 who have abandoned Swat, 80,000 are in government camps just south of the war zone.
In a statement, the military said militants were shaving off their beards and cutting their hair - flowing locks were fashionable among the Swat Taliban - in order to escape by mingling with the refugees pouring out of the valley. Authorities have placed security at the camps and say they are trying to stop any militants infiltrating them.
The army says it is advancing slowly to limit civilian casualties. Public opinion appears to support the offensive, but the mood could quickly turn against the pro-Western government if the fighting drags on and civilian hardship mounts.
While insisting it will win in Swat, Pakistan's army complains of a lack of equipment, including night-vision technology and helicopters - shortcomings that Washington has pledged to address.
U.S. forces are already training a Pakistani paramilitary force in the frontier region, considered the likely hiding place of al-Qaida chief Osama bin Laden.
A senior U.S. defense official said the Pentagon was considering plans to accelerate and expand that training.
U.S. and Pakistani officials are discussing a program that would increase the number of U.S. special operations trainers in the country and expand the schooling to the regular army, said the senior official, who spoke on condition of anonymity because the discussions are preliminary and no decisions have been made.
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