Flooding has displaced an additional 1 million people in Pakistan’s Sindh province in the past two days, according to new UN estimates released Friday.

“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 UN’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.

“The magnitude of this crisis is reaching levels that are even beyond our initial fears, which were already leaning towards what we thought would be the worst. The number of those affected and those in need of assistance from us are bound to keep rising. The floods seem determined to outrun our response,” he said.

On Thursday, authorities issued an evacuation warning after a breach in one of the embankments caused by rising water levels in the Indus River, said Ali Gul Sanjrani, a senior government official in Thatha district.

Sirens blared Thursday from mosques in Indus River delta cities and towns like Shahdadkot where remaining berms and levees holding back water were about to burst.

Floodwaters have started to recede across Pakistan, but in the Indus delta, the potential for more flooding remained high, especially given high tides in the Arabian Sea, where the Indus spills out.

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.

Many refugees have sought shelter at relief camps, where food and drinking water are now available. But every day, there are new camp arrivals — people who were already poor, who now have nothing.

The damage from Pakistan’s worst humanitarian catastrophe is sure to hurtle the impoverished nation back in terms of development. This week, America’s top aid official saw firsthand the dire needs in Pakistan.

Dr. Rajiv Shah, administrator for the U.S. Agency for International Development, said he was deeply moved by his visit to Sukkur and that aid agencies were “scaling up their response efforts as quickly as they possibly can.”

Shah announced the United States would be diverting another $50 million for flood relief from the Kerry-Lugar act, which allocated $7.5 billion in nonmilitary assistance to Pakistan over five years.

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– CNN.COM