Districts in Pakistan's Sindh province are on high alert for flooding after heavy rains devastated other parts of the country.
The reaction of the government has cast fresh doubts over the leadership of President Asif Ali Zardari, who is on a five-day visit to Britain.
At least 1,600 people have so far been killed by the flooding.
The National Disaster Management Authority (NDMA) said 12m people have been affected in two provinces, a figure that does not include southern Sindh.
Heavy rains are expected in areas already struck by the worst floods in 80 years.
Considerable damage is expected in mainly rural areas in Sindh after floodwaters moved down from the northwest and through the central agricultural heartland of Punjab.
'At least four districts are on high alert as the flood wave prepares to enter Sindh,' the UN Office for the Coordination of Humanitarian Affairs (UNOCHA) said.
UN officials said more than 500,000 people have been moved out of Sindh.
The floods have stoked popular anger at Mr Zardari, who went ahead with visits to Paris and London at the height of the disaster, which swallowed up entire villages.
Mr Zardari has rejected the criticism, telling the BBC's Newsnight programme that Prime Minister Yusuf Raza Gilani was dealing with the crisis and keeping him posted on developments.
Flooding is also taking a toll over the border in India, where rain was hampering rescue and relief efforts.
Flash floods have already killed at least 113 people in the Himalayan region of Ladakh.
Irish aid agencies have launched emergency appeals to help the survivors of the floods.
Concern has warned that the damage and suffering caused by the flooding could be worse than that caused by the earthquake in Pakistan five years ago.
Trócaire's emergency officer in Pakistan Cecil Dunne said infrastructure has been severely damaged and people are in desperate need of support.
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