Officials in Pakistan have said the number of people affected by severe flooding in the country has increased dramatically, more than doubling to 12m.
The National Disaster Management Authority said 650,000 homes have been destroyed and 500,000 people have been evacuated.
Pakistan President Asif Ali Zardari, who is on a state visit to Britain, has faced widespread criticism for not returning home to oversee relief efforts.
Pakistan is on red alert for extreme flooding in its rich agricultural south as the worst monsoon rains in 80 years continue to sweep through the country.
Authorities in the densely-populated southern province warned that major floods were expected in the next 48 hours in the fertile basin along the swollen Indus river.
The situation was described as a 'major catastrophe' by Manuel Bessler, who heads the UN Office for the Coordination of Humanitarian Affairs in Pakistan.
The meteorological office issued a red alert for the area overnight, warning of an 'imminent' and 'extreme' flood threat.
'At least 11 districts are at risk of flooding in Sindh, where more than 500,000 people have been relocated to safer places and evacuation still continues,' said the UN OCHA.
Sindh Irrigation Minister Jam Saifullah Dharejo said: 'Some 200,000 people have been evacuated and we are now forcing the people not willing to leave the area.
'Unprecedented rain has also hindered rescue and relief activities, but we are working with army and navy to avoid any loss of human life,' he said.
Flooding also spread to Indian-held Kashmir killing at least 60 people.
Floods washed away entire villages and killed at least 1,600, according to UN estimates.
UN special envoy Jean-Maurice Ripert said that a flash appeal was being urgently put together in response to the crisis.
He said there was a risk of epidemics because of lack of clean drinking water.
Click here for details of Irish aid agencies responding to the disaster
