A Pakistani army 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, the BBC reported Friday.

Added to more than 550 000 who had already fled, this threatened to create one of the world's biggest displacement crises, the UNHCR spokesman said.

Pakistani spokesman Gen Athar Abbas said the government had taken measures to provide for the civilians fleeing.

But reports suggest some civilians are being prevented from leaving militant-held areas.

Pakistani PM Yusuf Raza Gilani vowed on Thursday to "eliminate militants and terrorists" from Swat, a bastion of Taleban rule.

A full-scale offensive had begun on Friday, with helicopter gunships blasting militant strongholds from the air and troops conducting operations on the ground.

Displaced civilians have been forced to flee with very little preparation, aid workers say.

They say families have often been separated as they fled, and doctors in displaced camps reported many were suffering psychological trauma.

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.