Pakistan is broadening its offensive against about 5,000 militants
Pakistan's army says it has dropped troops by helicopter to tackle a Taleban stronghold as part of a broadening offensive in the north-west.
The troops were landed in the sparsely populated Peochar valley in Swat.
A BBC correspondent says Peochar is one of the bases of Swat Taleban chief Maulana Fazlullah.
Hundreds of thousands have fled the fighting and Human Rights Watch has urged the army and Taleban to avoid civilian casualties.
One resident in the main town of Mingora in Swat described the situation there as dire.
The man, who did not want to be identified, told the BBC that food supplies were running out, and electricity and gas had been cut off.
"Thousands of people are still trapped in Mingora, but the town is like a ghost city as no one dares to come out in the streets."
Up to 15,000 troops have now been deployed in the Swat valley and neighbouring areas to take on up to 5,000 militants.
An army spokesman confirmed to the BBC that troops airlifted by army helicopters had landed in Peochar but refused to reveal any more operational details.
Peochar is about 65km (40 miles) north-west of Swat's main town of Mingora.
The BBC's M Ilyas Khan in Islamabad says the area has camps both for combat training of militants and for training suicide bombers.
He says Maulana Fazlullah is normally based in his native village of Imamdheri in Matta but retreats to Peochar when under army pressure and is reported to be there now.
Our correspondent says there are also reports of the army blocking exit routes out of Peochar and if the area has been adequately cordoned off, the battle there may be fierce.
The BBC Urdu service managed to reach a civilian in Mingora by telephone and was told of "intermittent gunfire".
The man, who wanted to remain anonymous, said: "Thousands of people are still trapped in Mingora, but the town is like a ghost city as no-one dares come out on the streets.
Thousands of Pakistanis have fled to camps from the fighting
"Everybody wants to leave with no gas and food stocks running very low but the strict curfew by the government has given us no choice but to stay put."
Human Rights Watch has meanwhile urged the army and Taleban to do all they can to avoid civilian casualties in Swat.
Brad Adams, Asia director at Human Rights Watch, said: "Beheadings and use of human shields by Taleban forces are not a blank cheque for the Pakistani army.
"Winning the war, but also the peace, in Swat can only be achieved by minimising civilian suffering."
Human Rights Watch said it had reports of the Taleban mining parts of the Swat valley and preventing people leaving Mingora.
The UN has expressed fears for the 360,000 Pakistanis who have fled and has said it will deliver emergency humanitarian aid.
UN High Commissioner for Refugees Antonio Guterres said: "This is a huge and rapidly unfolding emergency which is going to require considerable resources beyond those that currently exist in the region."
Pakistan's military says it has killed hundreds of militants so far.
Pakistan's government signed a peace agreement with the Swat Taleban in February, allowing Sharia law there, a move sharply criticised by Washington.
The militants then moved out into neighbouring districts, causing further alarm.
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