Islamabad: The death toll from Pakistan's floods, which have officially claimed 1,600 lives, will rise as waters recede and the number of missing are counted, Pakistan's disaster agency said on Friday.

The country's worst ever natural disaster has affected more than 17 million Pakistanis, with more than eight million dependent on aid as rescue operations continue to evacuate swathes of the south still under threat.

Since the floods began a month ago, triggered by monsoon rains, the death toll has risen little from an early estimate of 1,500 people, but that is likely to change "significantly" once the seasonal rains end, said a spokeswoman for the National Disaster Management Authority (NDMA).

"There will be a rise... a significant rise because we will be in a position to find more bodies after these water levels recede," Amal Masoud said.

"There will be quite a number of missing people but it will not be as alarming as if we compare it with the 2005 earthquake," which killed 73,000, she said.

Of the 1,600 confirmed dead, 1,086 people were killed in the northwestern Khyber Pakhtunkhwa province, 183 in northern Gilgit-Baltistan district, 109 in southern Sindh, 103 in central Punjab, 71 in Pakistan-administered Kashmir and 48 in southwestern Baluchistan.

Flooding has also engulfed 20 per cent of the land, including huge areas of rich farming pastures on which Pakistan's struggling economy depends.

Maurizio Giuliano, spokesman for the UN Office for the Coordination of Humanitarian Affairs, said the number who died as a direct result of the floods was relatively limited because most people were evacuated or managed to escape.

"But that does not mean that the death toll over the longer-term would not be a concern because many more people risk dying in a second-wave of death," he said, referring to the threat posed by water-borne diseases.