Pakistan's worst flooding in nearly a century has now affected more than four million people and left at least 1,600 dead, says the UN.

While floods in the north-west began to recede, the vast body of water has been moving down the country into new parts of Punjab and menacing Sindh province.

All wells have been contaminated and water-borne diseases are spreading, officials say.

The region is midway through monsoon season and more rain is forecast.