A Pakistani offensive against militants in the Swat Valley has displaced some 200,000 people recently, the UN says.
A spokesman for the UN's refugee agency UNHCR said another 300,000 were already on the move or about to flee in extremely perilous circumstances, BBC reported.
As the fighting escalates, the UN said this threatened to become one of the world's biggest displacement crises.
Pakistan military spokesman Gen Athar Abbas says measures have been taken to provide for the fleeing civilians.
But reports suggest some civilians are being prevented from leaving militant-held areas.
Pakistani Prime Minister Yusuf Raza Gilani vowed on Thursday to "eliminate militants and terrorists" from Swat, a bastion of Taleban rule.
A full-scale offensive began on Friday, with helicopter gunships blasting militant strongholds from the air and troops conducting operations on the ground.
Despite now abandoned attempts to secure a peace deal in and around Swat, the area - close to the border with Afghanistan - has long been riven with tensions.
The UN estimates that before the current crisis, some 550,000 people had been displaced since last August, said UNHCR spokesman Ron Redmond.
Those displaced over recent days have been forced to flee with very little preparation, aid workers say.
They say families were often separated as they fled, and doctors in displaced camps report many are suffering psychological trauma.
Many residents are fleeing Mingora, the main town in Swat Valley, which was home to several hundred thousand people before the latest fighting began.
Locals say that most of the current fighting is centred in the Kabal and Charbagh areas of Swat, as well as Mingora itself.
Speaking to the BBC, Gen Abbas confirmed that the military's objective in this now fully-fledged offensive was to eliminate some 4-5,000 militants from the Swat Valley and neighbouring districts of Dir and Buner.
"It will be a drawn-out affair," he warned, "because the militants in Swat have had time to entrench themselves in the area, mix with the people, and through coercion, fear and using terror as a weapon, eliminate all those who supported the government."
He said militants were "making best use of the terrain, which is ideal country for any guerrilla warfare".
No reliable figures on casualties in the conflict are available, though one report quotes the Pakistani military as saying that more than 150 militants and "several" soldiers have died since the offensive began.
Hospitals in the conflict zone also report civilian deaths.
The BBC's Mark Dummett in Islamabad says that the government's superiority in terms of military hardware would ensure it easy victory in a pitched battle.
But this is not a pitched battle, he says: the Taleban are able to fight, disperse and regroup. The militants are well-motivated, he adds.
At present, our correspondent says, the government is confident it has public support for its military campaign - but this could easily be eroded if civilian casualties mount.
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