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 worst floods in the region 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.

"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.

"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.