Habiba's story is just one of tens of thousands in Sukkur where hungry and bewildered villagers have sought shelter

Habiba arrived in Sukkur in the early hours of the morning, after travelling nonstop for three days to find somewhere that had not been washed away by the floods. She left her village, Marakh Bijarani 40 miles away in Kashmore district, with 60 neighbours and relatives packed on to one tractor and trailer, with a few clothes, cooking pots and bedding piled underneath them.

Only half the village managed to escape – those who had taken refuge on the raised bank of a dyke at the moment when the water suddenly rushed in. Habiba made it out with two young children, but she has no idea what happened to her husband and five other children.

"I haven't been able to contact my husband. We have no news on the others," said 40-year-old Habiba, who left her remote home district for the first time in her life when the floods came. "We are now at God's mercy."

"The water came suddenly, when a dyke broke. There was a lot of water. Maybe six, 10 feet. Our houses are gone," said Abdul Sattar, part of the group. They were lent the tractor by the local landowner.

The story of these hungry, bewildered villagers is now commonplace in Sukkur, where tens of thousands of suddenly destitute people, a tide of misery, have arrived from the countryside.

The full scale of the tragedy engulfing Pakistan is now emerging, with the United Nations saying today that the floods, which have entered their third week, have now affected 13.8 million people. This is more than the combined victims of the three most recent big natural disasters the world has faced – the Haiti earthquake in January, the 2005 earthquake in northern Pakistan, and the Asian tsunami – although the death toll from the flooding is much lower. Officially it stands at around 1,600, but with so many parts of the country inaccessible, it is feared it could end up being much higher.

In Sukkur many of those stranded by the floods are now sleeping by the sides of roads, under bridges, on the town's bypass and at the railway station. It is oppressively hot and humid, and mosquitoes plague them through the night.

Habiba and her group of fellow villagers are among the relatively lucky ones – they are camped in a school building near the airport after a school caretaker took pity on them. But they have no food and no money, and conditions are squalid.

Trucks and tractors arrive constantly, laden with sacks, beds and mattresses piled high. The human cargo, usually the women and children, perch precariously on top. Some wander into the outskirts of town with their herds of buffalo, their most valuable possession, having walked for days. Shepherds carry shotguns strapped to their bare backs, to ward off bandits.

Sindh province was the last area to be hit by the flooding, as the waters tumbled south across Pakistan, beginning to reach here in a torrent over the weekend. The situation in Sindh, where a large water barrage is also under threat, is now a serious concern to aid agencies and the government.

Under a highway flyover, by the Arain railway station in Sukkur, several hundred people are living in the mud by the sides of the road, many spending their third night out in the open. They have no water or food, with only charity from passing townsfolk keeping them going. They say that they were turned away from government relief camps, which were full.

A newborn baby died by the side of the road today, according to one.

"The ministers, the VIPs pass this spot in their motorcades, but they don't stop," said Shaukat Ali, 45, who had arrived with around 200 people from a village near Ghouspur, in Kashmore district. They were all now living under the flyover, which provided scant shelter from the rain. "If they're not going to give us anything else, at least give us drinking water."

According to Ali, the whole of Kashmore district was under water because the authorities had deliberately flooded it by cutting a hole in the dyke – in order, he claimed, to save the land of a government minister from being inundated. Ali said they had just two hours' notice from the time the dyke was breached to get out. Whether or not the charge was true, it was firmly believed, and causing great anger.

In order to relieve flooding pressure, the authorities are believed to be creating outlets in dykes to empty water on to farm land, though the government has not talked publicly about the practice. Because of the protective dykes, many had stayed in their villages, believing that their homes could not be flooded. But, by Saturday night, they had been proved horribly wrong, as dyke after dyke was breached.

"They [the government] are taking all the aid for themselves. They're pocketing it. There's nothing coming to the people," said Mukhtiar Ali, Shaukat's brother. "The government has done nothing for us. No medicine, no food, no water, no tents, no blankets."

At one of the official aid centres, the government degree college, around 1,500 people have found shelter. The classrooms now have large families camped in them, perhaps 100 crammed into one of the rooms, with just sheets to cover the floor. Young children sit around lifelessly or cry out. Officials at the centre admit that food, water and sanitation fall far short of requirements. "We're all trying to do our best," said the college principal, Bashir Ahmed Ranejo.

There are only 20 government camps in Sukkur, but up to 300,000 refugees have overwhelmed the city. On the northern tip of Sindh, it is now a focal point for the still growing disaster.

Some 300,000 homes in all four provinces of Pakistan have been washed away by the raging waters, as 10,663 villages have been affected – including 2,500 villages in Sindh.

"At the moment Sindh is the biggest concern. It is still developing. Some other areas, like parts of KPK [Khyber Pakhtunkhwa, the North-West Frontier province] have stabilised," said Maurizio Giuliano, a spokesman for the UN.

However, rescue workers have been unable to reach up to 600,000 people marooned in the north-west Swat Valley, where many residents had been trying to recover from a Taliban takeover which was only beaten back last year.

"The magnitude of the tragedy is so immense that it is hard to assess," said prime minister Yousaf Raza Gilani during a visit to the city of Multan, in a flood-hit part of Punjab province.