Christian charity World Vision says it fears an increase in water-borne diseases among flood victims as fresh rains compounded relief operations in Pakistan.

World Vision said diseases such as diarrhoea and cholera were rampant and that an increasing number of children were suffering from skin diseases and eye infections.

The development agency has opened an emergency primary health clinic in Lower Dir, an area of Pakistan severely hit by monsoons and floods.

The clinic has received an influx of patients from the area who are showing up "parched and hungry", having walked through rain for hours, according to Dr Sheraz Iqbal, a physician with World Vision in Pakistan.

"They’re suffering from ailments such as diarrhea and acute respiratory infections, which can become life-threatening unless treated. The situation is desperate,” he said.

In addition to emergency health services, the aid agency is working to distribute water, food items, mattresses, bed sheets, and kitchen kits.

World Vision plans to reach 150,000 people over the next three months of its response.

However, the agency says relief workers' efforts continue to be hampered by ongoing rainfall, damaged roads, and a shortage of boats. Fresh showers on Sunday reportedly worsened the flooding.

According to official statistics, the floods have affected over 14 million people including an estimated 1.4 million children across the country. The local media estimates that at least 1,700 have been killed.

Pakistani Prime Minister Yousuf Gaza Gilani issued an appeal for more international aid, saying the damage was "far greater" than the 2005 earthquake that killed about 70,000 people.

"Millions of people have suffered and still there is more rain and further losses are feared. I appeal to the world to help us, we are doing what we can," Gilani told reporters.

More than 250,000 homes have reportedly been damaged or destroyed across Pakistan.