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The floods that have left millions homeless in Pakistan are still wreaking havoc, destroying crops, hindering efforts to get aid to refugees and threatening the country's few remaining levees.

Three towns in the southern Sindh province's Thatta district remain in danger from the surging waters of the Indus River and officials were urging thousands to leave the area on Sunday.

At least two levees along the river are potential trouble spots and are being strengthened, said Hadi Bakhsh Kalhoro, an official with the Sindh provincial Disaster Management Authority.

"We are hopeful the flood will pass on to the delta without creating much trouble here," he said.

In many areas, ordinary Pakistanis set out by themselves to strengthen levees with sandbags and stones.

"We are doing it on our own," Thatta resident Munawar Ali told Dunya TV. "We are not seeing any government agency here to help us save our lives and our belongings."

David Toycen, president and CEO of World Vision, said aid workers are struggling to get the most basic necessities to victims of the flood.

"It's things like water, tents, food, basic medical supplies -- the things that basically keep people alive," he told Â鶹ӰÊÓ Channel on Sunday. "We're really concerned now that we've had children who've been in wet clothing for day after day with no shelter. So there's signs of pneumonia, diarrhea, even one case of cholera reported."

The continuing flooding is making it nearly impossible to get help to some parts of the country, Toycen said.

"It's been very problematic. Roads are out; it continues to rain; bridges are out; so transportation is difficult.," he said.

"It's a race against time, especially when the flooding continues in so many places … We just do our best."

But Toycen said relief agencies have been buoyed by news that more aid funds are beginning to flow into the country, including an announcement Sunday that Canada's federal government will now match private donations to flood relief.

More than $800 million in international aid has been delivered or promised, Pakistan's foreign minister said Sunday adding that his country was grateful for the assistance.

"The total commitments and pledges that Pakistan has got so far are $815.58 million," Foreign Minister Shah Mahmood Qureshi told reporters in Islamabad. "In these circumstances, when the West and Europe and America are going through a recession ... this kind of solidarity for Pakistan, I think, is very encouraging."

Slow response to crisis

Aid flowed relatively slowly to Pakistan in the first weeks of the crisis, apparently in part because many were unaware of the vast scope of the damage and the death toll was relatively low -- around 1,500 people.

Across the flooded regions, thousands have decided to stay in their homes, often sleeping on rooftops to stay above the rising water.

Stranded on tiny islands a few inches above the water line and refusing offers of rescue, they reflect Pakistan's widespread poverty and a collapse in public confidence in their government's willingness to protect them.

"The women were scared before we sent them away, and we're scared now," said Mohammed Ayoub, one of about 30 men who remained as guardians and to build up the embankments in case of more flooding.

About 400 villagers have already fled. "How can we all leave?" he asked. "We have to stay here if we want to protect what we own."

Although the worst of the flooding has now passed in their part of Sindh province, life is desperately miserable: There is little food, no electricity and the village well is filled with brown flood water.

While doctors say cases of malaria and gastrointestinal diseases are spiking across the flooded areas, and there have been sporadic cases of cholera, there are enough fishing boats to allow people to flee to the shoreline if they want.

The floods began in late July in the northwest after exceptionally heavy monsoon rains swelled rivers to overflowing. Within a few days, as much rain fell as the country normally receives in a year.

But that was only the beginning. The rain that had fallen in the mountainous northwest began flowing south through the plains, swelling rivers, breaking through embankments, flooding an area the size of Italy and wreaking havoc across the agricultural heartland.

The deluge has affected about one-fifth of Pakistan's territory, straining the civilian government as it also struggles against al Qaeda and Taliban violence.

At least 6 million people have been made homeless and 20 million affected overall.

Pakistan can ill afford the crisis. The South Asian country's economy was already being kept afloat by billions of dollars in loans from the International Monetary Fund, and the cost of rebuilding after the floods will likely run into the billions.

The IMF said it will meet with Pakistani officials this week to discuss the floods and what the country must do to cope.

With files from The Associated Press