ISLAMABAD -- Rising numbers of civilians are pouring out of Pakistan's war zone to flee battles between soldiers and Taliban militants but the fate of those left behind is uncertain, humanitarian workers say.

"How much civilians are affected, we don't know, and for that we need access," said Billi Bierling, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs in Pakistan.

Up to 250,000 people have fled the military's major offensive, now into a third week in South Waziristan on the Afghan border, said Lieutenant General Nadeem Ahmad, chief of the state-run agency handling the displaced.

But no one knows the exact number of displaced people or those left in the conflict zone because foreign aid workers have not been able to enter the areas, the humanitarian workers say.

"We know that there are still civilians trapped in the areas where fighting's taking place," said Sebastien Brack, spokesman for the International Committee of the Red Cross (ICRC) in Pakistan.

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

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

The military said that only one to two percent of the population remained in the conflict zone.

"These are people who have stayed back to take care of their properties," Ahmad told AFP.

But the ICRC warned that the numbers were likely higher because others could not afford the "extortionate" rates charged by people offering transport for those fleeing.

"The people who are left behind are often the poorest of the poor," he said.

Like aid workers, reporters usually have no access to the conflict area, where communication lines are down.

A correspondent briefly invited by the military into areas of South Waziristan under its control on Sunday saw no civilians.

Brack said the ICRC had tried unsuccessfully to gain access to South Waziristan to assist victims of fighting and to visit detainees, in accordance with its mandate.

US-based Human Rights Watch has urged Pakistan to ensure that sufficient supplies reach trapped civilians, warning of "catastrophe" without aid access.

People fleeing the area have reported food shortages and rising numbers of civilian casualties, HRW said.

"International humanitarian agencies have not been given access to South Waziristan and this is a problem," said Ali Dayan Hasan, senior South Asia researcher for the watchdog.

HRW has called on both the military and Taliban militants to take all necessary precautions to avoid civilian casualties. Both sides are obliged to abide by international humanitarian law, it said.

Although ICRC has no staff in the area, it has for months been sending medical supplies to assist eight clinics in Waziristan, Brack said.

Those clinics are each treating two to three patients injured in the fighting each day, he said, but "it's getting more and more difficult to get medicine to them".

Ahmad said more than 400 tons of army rations had been allocated for people still in the area of military operations.

"I would be hesitant to take the Pakistan military's word on anything," said Hasan of HRW. Without independent monitoring, "how do we know that 400 tons of rations has actually got to the civilians?"

Those escaping the fighting 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, Ahmad said.

The United Nations has no international staff in those towns but is working through local partners to distribute food, blankets and other essentials.

"We are certainly meeting the needs of the people who have arrived in Dera Ismail Khan and Tank," while the UN negotiates with the government to allow its expatriate staff to join the emergency relief effort, Bierling said.

The government says it does not want international organizations in the area because of concerns for their safety.

The United Nations on Monday said it was pulling out non-essential international staff from North West Frontier Province because of what it called "the intense security situation".