A massive outbreak of waterborne diseases and deadly snakebites are weighing on hundreds of thousands of flood survivors in Pakistan, officials said on Sunday, as the death toll from the months of devastation rose to 1,300.

At least 26 more people died on Saturday, most in the southern province of Sindh, which has been hit the worst and faces another deluge from Pakistan’s longest river that is swollen by rainwater flowing downstream from the mountainous north.

The latest deaths have taken the toll since June's start of the climate-induced catastrophe to 1,290, the national disaster agency said in the latest update.

Several towns and cities in Sindh that remained submerged for almost two months now face another deluge from the Indus River, provincial government spokesperson Murtaza Wahab said.

A massive rescue operation backed by helicopters and boats was underway for a third day to evacuate hundreds of thousands of people marooned in remote villages, Wahab added.

At least half a million survivors living in makeshift shelters without access to clean drinking water, toilets and sanitation are now facing waterborne diseases, provincial Health Minister Azra Pechuho said.

More than 100 cases of snake bites and nearly 500 of dog bites have been reported from the flood-hit areas, Pechuho said.

Medics from the military, government hospitals and charities are treating nearly 150,000 patients for diarrhoea, more than 100,000 with skin infections and thousands with malaria symptoms at temporary clinics, the minister added.

More rains have been predicted on the northern mountains through Tuesday.