At least 11 UN workers have been killed in Pakistan this year, and fears of further attacks have increased over the past two and a half months.
The organisation started to review its operations after an October attack on the World Food Programme office in Islamabad killed five people.
A UN official speaking on condition of anonymity told the Associated Press news agency that security managers are seeking a reduction of up to 30 per cent in the
UN's international staff working inside Pakistan.
An undetermined number of national staff will likely be moved out of Pakistan's North West Frontier Province and from the western province of Baluchistan, the official said.
The world body employs about 250 international and 2,500 national staff in Pakistan.
UN operations in Pakistan since early 2009 have grown to about $1bn for the nation's "sustainable development" needs, officials said. Since spring they have also handed out some $475mn in emergency humanitarian aid in northern Pakistan.