BBC NEWS - Fierce fighting is taking place between Pakistani troops and Taliban militants in Mingora, the main city in the militant-controlled Swat valley.
At least 17 militants have been killed in the clashes, the army says. The Taliban deny the deaths.
The push into Mingora is seen as a key phase of an offensive aimed at crushing the militants, whose influence extends across a wide area of the north-west.
The fighting began after a peace deal broke down earlier this month.
"Street fights have begun," Maj Gen Athar Abbas told reporters.
He said soldiers had cleared parts of the city, but added that the pace of the offensive was "painfully slow".
"This is an extremely difficult, extremely dangerous operation, because clearance has to be done street by street, house by house."
The military says the city is surrounded, most of the militants' ammunition dumps are destroyed and their supply routes cut off.
The BBC's Shoaib Hassan, in Islamabad, says it is the most important battle yet in the army's offensive against the Taliban in Swat.
Click to view image: 'Pakistan & Taliban movements'
A swift victory would bolster public support for a greater fight against the militants, our correspondent adds.
A Taliban spokesman confirmed that the military had entered Mingora, but denied that any militants had been killed.
The spokesman also said the Taliban would fight the security forces to their last breath.
Residents say the militants are still in control of the city.
Nearly 1.5 million people have been displaced by this month's fighting in the north-western region, and about two million since last August, the United Nations refugee agency says.
One resident who fled the Mingora area told the BBC that he was among many who had lost everything.
"Our homes were destroyed - we left behind our cattle and our properties," he said. "We walked all the way and had to walk for two days on the mountains."
On Friday, the UN appealed for $543m in humanitarian aid to help those displaced by the conflict.
Pakistan's army began an offensive against the Taliban on 2 May after the peace deal broke down and the militants began expanding their area of influence.
A recent investigation by the BBC suggested that less than half of Pakistan's North West Frontier Province (NWFP), which contains Swat Valley, and the neighbouring Federally Administered Tribal Areas is under full government control.
In Swat, the army says that about 15,000 members of the security forces are fighting between 4,000 and 5,000 militants.
It says more than 1,000 militants and more than 50 soldiers have been killed since the offensive began.
Tags: Pakistan, army, Taliban, Mingora, militant-controlled, Swat Valley, street fighting
Location: Mingora, North-West Frontier, Pakistan (load item map)
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