The UN refugee agency said nearly 1.5 million people have fled their homes in Pakistan this month, saying that fighting between government forces and Taliban militants is uprooting more people faster than probably any conflict since the Rwandan genocide of the 1990s.
"It has been a long time since there has been a displacement this big," said UNHCR spokesman Ron Redmond, trying to recollect the last time so many people were uprooted in such a short period.
"It could go back to Rwanda," Mr Redmond said, referring to the 1994 massacre of ethnic Tutsis by the majority Hutus in the African country. "It's an enormous number of people."
Mr Redmond spoke as the UN's refugee chief Antonio Guterres returned from a three-day mission to Pakistan, and as Taliban forces vowed to resist military advances in the northwestern Swat Valley until their "last breath".
He said the newly uprooted added to over 550,000 people who were already registered as displaced in northwest Pakistan, meaning there are over two million people separated from their homes in the country.
"Humanitarian workers are struggling to keep up with the size and speed of the displacement," Mr Redmond told
journalists in Geneva, where UNHCR has its headquarters.
He said a lack of help for the displaced and the many thousands of families hosting them could cause more "political destabilisation" for the country.
The UN believes around 15 to 20 per cent of the displaced are in camps at the moment - around 250,000 in some 24 camps, UN humanitarian chief John Holmes said, "which means most people are either with host families, communities, in rented accommodation or somewhere else.
"The situation is volatile and changing rapidly," Mr Holmes told reporters at UN headquarters in New York.
The International Organisation for Migration was sending trucks full of quilts, sleeping mats and other goods to help the influx of people in camps near the city of Peshawar, said spokesman Jean-Philippe Chauzy.