"We have more people on the move, to whom we need to provide relief. An already colossal disaster is getting worse and requiring an even more colossal response," said Maurizio Giuliano, a spokesman for the U.N.'s Office for the Coordination of Humanitarian Affairs.

Giuliano said rains have forced the evacuation of an estimated 1 million people in southern Sindh in the past 48 hours or so.

The U.N. also said on August 27 that it is increasingly concerned about flood-driven malnutrition among children.

Acute malnutrition was high in much of Pakistan even before the floods. For instance, 27 percent of children under 5 in Baluchistan province were malnourished, as were 17 percent of children in Punjab, according to the U.N.

Already, more than 17 million Pakistanis -- from the Chinese border in the north to the mouth of the Indus in the south -- have been affected by the monsoon floods that began a month ago.

To date, Pakistan's unfolding tragedy has claimed 1,600 lives, according to the National Disaster Management Authority. That number is likely to rise as more drowned bodies are discovered in receding waters.