On the road in from the airport, the water shimmered under the moonlight as men, women and children sat in the dark, near the would-be lake shore. During the day, river dolphins can usually be spotted in the nearby river.

Idyllic, you might think. But this dusty and ramshackle town is at the front line of one of the world’s worst humanitarian disasters in living memory, sitting on the Indus River in Pakistan’s southern Sindh province.

Usually there is no water lapping up at the roadside, and the only people there would be those out for an evening snack after the daytime Ramadan fast. But since torrential monsoon rain swelled the Indus River and of Pakistan has been left flooded  and 4 million Pakistanis are now homeless, and another 600,000 are threatened down-river, meaning they might have to flee as well.

In the city, I spoke to Ashraf, who said he had left his family outside the city where he had come to buy some food. “We managed to gather up some of our possessions before the waters came, but we did not have much warning. Our home is under water completely. I have enough money on me to feed my children for another couple of days, that is all.”

Pakistan is once more reeling from nature’s cruel hand, less than five years after an earthquake killed 80,000 people mainly in the country’s north. This time — though the death toll is much lower — the impact is vast — running the entire 3180km length of the Indus River from the mountainous north of Pakistan to the flood-prone plains here in the south.

More than 6.8 million hectares of land is under water and, out of the mind-boggling 20 million people affected by the floods — about 800,000 people remain beyond the reach of aid workers or the Pakistani army, cut off by rising waters that in turn wrecked bridges and submerged roads. Cases of diarrhea, cholera, skin diseases, as well as malaria and dengue — with mosquitoes proliferating amid the floodwaters — are growing.

Almost 5 million people now have no access to clean water. The disaster is as vast as the swollen country-long lake that the Indus River has become.

Sukkur is derived from the Arabic word for intense, according to some historical accounts that date the name to Umayyad conquerors coming to the region more than a millennium ago. For aid workers with the NGO GOAL now stationed in the town, the epithet seems apt.

Emergency co-ordinator Brian Casey was at the forefront of relief work in Haiti after the earthquake and in Burma after Cyclone Nargis. He said: “People are hungry, people are getting sick, and we don’t know yet how much worse things will get as the water rises in places. And at the same time we have to think about how to help people rebuild homes and farms once the waters recede.”