Continuing floodwaters in Pakistan engulfed a town of 250,000 on Sunday while threatening others in the proximity.
The town of Sujawal is now swamped beneath at least five feet of water and up to 10 feet in some areas after floodwaters broke through a levee on the Indus River.
Most of the city's residents evacuated before the waters rushed in and no deaths have been reported so far.
"We fled so hastily that we could not even pick up our belongings," Sujawal resident Amena Bibi told the BBC.
Sujawal's neighbors, including the 350,000-populated city of Thatta, are now under imminent threat of flooding although officials are trying to prevent the water from spreading by building makeshift levees.
A majority of Thatta's residents have already fled the region.
The damage is the latest in nearly a month's worth of flooding that observers say is Pakistan's worst natural disaster in the last 80 years.
Pakistan authorities have estimated that as many as 20 million people have been affected by the flood, and the United Nations says 8 million people are in need of emergency assistance, including 800,000 that are only reachable by air.
A lack of dry land routes has complicated humanitarian assistance and exacerbated the effects of what victims have criticized as an inadequate relief effort.
Hundreds of thousands, including many that fled from Sujawal and Thatta, remain without shelter or food, while aid money continues to trickle in much slower than relief groups say is necessary to prevent further catastrophic damage.
A $460 million appeal launched by the U.N. in early August is now 60 percent funded.
"These unprecedented floods pose unprecedented logistical challenges," said John Holmes, U.N. Emergency Relief Coordinator, "and this requires an extraordinary effort by the international community."
On Saturday, the United States said it would provide an additional 18 helicopters to help the effort. The U.S. has already provided fifteen helicopters and three aircraft.
Earlier in the week, the U.S. committed $50 million to the U.N.'s disaster relief fund, adding onto their previous contribution of $150 million.
USAID chief Rajiv Shah said on Wednesday that the results of the country's aid have been "validated" and that the U.S. will "continue to stand by the people of Pakistan during this hour of humanitarian need."