Up to 250,000 people have fled a tribal region on the Afghan border where the Pakistani army is in the third week of a major offensive against the Taliban, an official said Sunday.

The figure is higher than the 200,000 which the army reported had fled the conflict zone in South Waziristan last week.

Lieutenant General Nadeem Ahmad, chief of the government's Special Support Group, told reporters that between 244,000 and 250,000 people have migrated to the northwestern towns of Dera Ismail Khan and Tank, where they are staying with relatives, friends, host families or in rented houses.

South Waziristan is closed to reporters and aid workers.

A US-based rights group on Thursday urged Pakistan to ensure that sufficient supplies reach civilians trapped by the offensive, and warned of "catastrophe" without aid access.

Ahmad told AFP the army has allocated 405 tons of rations for the one or two percent of the population who stayed behind to look after their property.

Normally about 300,000 people live in the northern part of South Waziristan which the military seeks to clear of "terrorists".

The district is part of the lawless tribal belt where US officials say Al-Qaeda and their allies are plotting attacks on the West.

Around 30,000 troops are taking part in the offensive against an estimated 10,000-12,000 militants in the semi-autonomous region.