Hundreds of hungry families blocked a highway in Pakistan's flood-hit south on Wednesday, demanding the government provide more food as the UN warned of a "triple threat" to desperate survivors. Up to 500 people from a government-run relief camp in Thatta city, in the worst-hit province of Sindh, blocked the main road between Karachi city and Thatta for three hours calling for the state to provide food and shelter. "No food or water has been provided to us for the past two days," Mohammad Qasim, a 60-year-old resident of the flooded town of Sujawal, told AFP. The World Food Programme issued a stark warning of the threat to food supplies after a month of catastrophic flooding that has affected 18 million people, as the deluge flows south on its way to the Arabian Sea. Pakistan's government -- widely painted as corrupt and bogged down in red tape and infighting -- has been derided in domestic media over its response to the floods and has been the focus of angry isolated protests by the affected. While the international community has now donated 700 million dollars, domestic anger has been mounting at the civilian government, which has staggered from crisis to crisis in the 30 months since its election. "There is a triple threat unfolding as this crisis widens and deepens," World Food Programme chief Josette Sheeran said at a press conference in Islamabad on Tuesday, after visiting flooded areas. The triple threat was people's loss of seeds, crops and incomes, "leaving them vulnerable to hunger, homelessness and desperation -- the situation is extremely critical", she said. Devastation to farmland and transport links mean that food prices have rocketed, fanning frustration among the masses already struggling to make ends meet in Pakistan's shaky economy. In televised comments Wednesday, Prime Minister Yousaf Raza Gilani told his cabinet that the flooding had "devastated the infrastructure on a large scale and the government is trying to cope with the crisis". The floods have engulfed a fifth of the volatile country of 167 million, with 3.4 million hectares (8.4 million acres) of rich farmland ruined, according to latest UN figures. "We need to bring in a lot more (food). We're still looking at a caseload in urgent need of about six million but with the floodwaters still moving it's quite possible that number will increase," WFP spokesman Marcus Prior told AFP. Meanwhile, floodwaters flowing south in Sindh province entered one town and threatened another on the east bank of the swollen Indus. "Water has entered the outskirts of Jati town and is two kilometres (1.25 miles) away form Choohar Jamali town," senior city official Hadi Bakhsh Kalhoro told AFP, adding that a few thousand people remained trapped in both towns. Kalhoro said power cuts were hindering rescue efforts but said that all other districts in the southern province were now safe. Pakistani troops and city workers managed to save Thatta from the waters by fixing a breach in river defences on Tuesday, with most of the population of 300,000 now returned home, according to officials. Sindh is the worst-affected province, with 19 of its 23 districts ravaged as floodwaters have swollen the raging Indus river to 40 times its usual volume. One million people have been displaced over the past few days alone. River management official Qadir Bakhsh Palijo said that waters in the area were receding, but could take up to 10 days to lower to a "satisfactory level". Pakistan's government has confirmed 1,645 people dead and 2,479 injured but officials warn that millions are at risk from food shortages and disease. The UN children's fund UNICEF said the disaster had affected nearly 8.6 million children, with the risk of more deaths from waterborne disease if clean water, good nutrition, sanitation and vaccination are not forthcoming.