More heavy rain in Pakistan is frustrating efforts to help about 14 million people affected by severe flooding in much of the country.
Helicopter missions in the north-west have been grounded and a red alert has been issued for the south.
One dam in Sindh province has been breached and engineers are warning that the huge Tarbela and Mangla dams are close to their maximum levels.
The region's worst floods for 80 years have killed at least 1,600 people.
Pakistan's meteorological office has warned that at least two more days of rain are expected in Sindh, where authorities have declared an "imminent" and "extreme" flood threat.
Further downpours are also forecast in the badly-hit north-western province of Khyber-Pakhtunkhwa.
The BBC's Aleem Maqbool says places where floodwaters had receded are now submerged again, and roads that were opened are now closed.
"Things are getting worse. It's raining again. That's hampering our relief work," said UN World Food Programme spokesman Amjad Jamal.
Many helicopter aid flights in the north-west have been grounded by the bad weather.
The helicopters are essential in the region's rugged terrain because the floods damaged or destroyed most of the bridges, cutting off many survivors from relief. In the Swat valley alone, 29 bridges have been swept away.
"The situation is bad, particularly in the Swat valley, and we have advised people in low-lying areas to vacate their homes as river water levels are rising," said Adnan Ahmed, an official with Khyber-Pakhtunkhwa's government.
The deluge has brought the water levels behind the Tarbela and Mangla dams - two of the world's largest - dangerously close to their maximum, engineers warned.
A dam in northern Sindh's Kashmore district has already been breached, inundating large parts of the surrounding area with floodwater and forcing thousands of residents to take shelter on rooftops or in trees.
One man told the BBC said his entire village had been destroyed and all its livestock washed away.
The floods, brought on by seasonal monsoon rains, began in the north-west, but have now inundated a stretch of Pakistan about 1,000km (600 miles) long, primarily along the Indus river and its tributaries. Heavy rains in Afghanistan are also expected to swell the Kabul river, which flows into northern Pakistan.
With the flood surge heading south, authorities have evacuated more than half a million people living near the Indus as hundreds of villages have been inundated by floodwaters.
Some people, however, refused to leave their crops and homes.
"Let the flood come. We will live and die here," Dur Mohammed, 75, told the Associated Press in the village of Dadli.
Officials say 650,000 homes have so far been destroyed across the country, 1.4m acres (557,000 hectares) of agricultural land have been flooded and 10,000 cows have died.
Prime Minister Yousuf Raza Gilani's appeal for international aid has been answered, with many countries offering assistance.
The US has diverted helicopters and troops from Afghanistan to deliver aid while Nato has said it will co-ordinate supplies from member countries and partners.
A Nato spokeswoman said it was providing food, mosquito nets, tents, generators and medicines.
The UK's first aid flight arrived in Rawalpindi late on Saturday, with 500 tents to join the 2,000 already distributed.
The head of the British development mission in Pakistan, George Turkington, told the BBC that it had a team of engineers in the north-western city of Peshawar "to look at establishing an emergency bridge-building programme so we can contribute to opening up new access routes".
He said the priorities were water, sanitation equipment, food and shelter.
Our correspondent says much more is needed, with the renewed rains submerging new areas every day.
Meanwhile, President Asif Ali Zardari has rejected criticism that he should return from an extended foreign trip.
In a BBC interview, Mr Zardari said the cabinet was directing relief efforts, and he was being kept up to date about the situation.
"I'm the one who's given all the powers from the presidency to the parliament. The parliament is in session - the Senate is in session. It's the prime minister's responsibility, and he's fulfilling his responsibility."