KARACHI, Pakistan — Floodwaters inundated a large town in southern Pakistan on Sunday as authorities struggled to build new levees with clay and stone to prevent one of the area's biggest cities from suffering the same fate.

Almost all of Sujawal's 250,000 residents fled the town before the water rushed in, but the damage to homes, clinics and schools added to the widespread devastation the floods have caused across Pakistan, said Hadi Baksh, a disaster management official in southern Sindh province.

The floodwaters also threatened Thatta, a historic city of some 350,000 people who have mostly fled to higher ground. Thatta is the base of operations for local authorities trying to cope with a disaster that has overwhelmed the Pakistani government and international partners who have stepped in to help.

Authorities rushed to build makeshift levees across the road connecting Sujawal and Thatta, parts of which were already flooded, Baksh said.

"We are trying to plug the bridges at three different points to stop the water flow toward Thatta," said Baksh. "We are trying all our best efforts."

The floods began in the mountainous northwest about a month ago with the onset of monsoon rains and have moved slowly down the country toward the coast in the south, inundating vast swaths of prime agricultural land and damaging or destroying more than 1 million homes.

More than 8 million people are in need of emergency assistance across the country.

The United Nations, the Pakistani army and a host of local and international relief groups have been rushing aid workers, medicine, food and water to the affected regions, but are unable to reach many people.

The U.S. said Saturday that it would deploy an additional 18 helicopters to help with the relief effort. The U.S. military is already operating 15 helicopters and three C-130 aircraft in the country, the U.S. Embassy said in a statement.

The floodwaters that hit Sujawal on Sunday surged into the town after breaking through a levee on the swollen Indus River two days earlier. The town is about 150 kilometres southeast of Karachi, the capital of Sindh, and 25 kilometres southeast of Thatta.

Authorities in Sujawal were trying to limit the flood damage, but the water level has already risen up to 1.5 metres in the center of town and 3 metres in the surrounding villages, said Anwarul Haq, the top official in Sujawal.

Many of the people who fled Sujawal and Thatta headed to Makli, a hill just south of Thatta that contains a vast Muslim graveyard.

About half a million flood victims are camped out on the hill, Baksh said. Most lack any form of shelter and are desperate for food and water.

"We don't have water to drink, not to mention food, tents or any other facility," said Mohammed Usman, a laborer who fled Sujawal several days ago and needed water to help cope with a painful kidney stone.

On Saturday, flood victims blocked a road in Thatta to protest the shortage of aid, most of which is randomly thrown from trucks into crowds of needy people.