Associated Press
Pakistani flood survivors look out from a make-shift tent after fleeing their village of Sajawal near Hyderabad on Friday. U.N. officials estimate 6 million Pakistanis may be homeless due to flooding.

SUKKUR, Pakistan — Floodwaters that have reached the Indus River delta displaced at least 1 million southern Pakistan residents in recent days, U.N. officials said Friday, a significant escalation of what is already the country's worst natural disaster in its history.

Further upstream in central and northern Pakistan, floodwaters have begun to recede a month after record monsoon rains swept away roads, bridges and other infrastructure and left millions of people homeless. The death toll stands at 1,600.

In southern Sindh province, however, the floods continue to wreak havoc as they reach cities near the Indus delta. U.N. officials said the floods have forced the evacuation of 1 million people in the last two days, mostly from the Qambar-Shadadkot and Thatta regions in the southern end of the province. In terms of surface area affected by flooding, Sindh is now the hardest-hit of Pakistan's provinces.

"An already colossal disaster is getting worse, and requiring an even more colossal response," said U.N. spokesman Maurizio Giuliano. "The magnitude of this crisis is reaching levels that are even beyond our initial fears."

Officials with the world body say the speed with which the crisis is spreading is outpacing the international community's efforts to reach legions of flood victims who lack access to food, clean drinking water, shelter and health care. U.N. workers are providing drinking water to 2.5 million people but have yet to reach the estimated 3.5 million others still in need.

Relief workers are especially concerned about the risk to children, many of whom were already in poor health before the floods. U.N. officials estimate that at least 70,000 children under the age of 5 and living in flood-affected areas suffered from acute malnutrition before the crisis. Up to 20 percent of children in flooded regions are suffering from diarrhea-related disorders and at high risk of dehydration and malnutrition.

Complicating matters is the threat Thursday by Pakistani Taliban, who have hinted they might attack the foreign aid workers. The militant network has a history of attacking aid groups, including agencies under the U.N. umbrella. Militant spokesman Azam Tariq said the U.S. and other countries were not focused on providing aid to flood victims but had other motives he did not specify.

On Friday in the U.S., the chairman of the Joint Chiefs of Staff said the threat against aid workers helping flood victims in Pakistan must be taken seriously and shows that insurgents have no moral boundaries.

"I share the concern that was stated yesterday by the State Department," U.S. military chief Adm. Mike Mullen told reporters after a luncheon speech to the City Club of Cleveland in Ohio.

Mullen said "significant precautions" have been ordered to protect U.S. forces supporting the relief effort.

"We would hope that all of those who are providing aid in this very difficult set of circumstances would certainly not be impeded with respect to that," he said.

Contributing: Associated Press