The Pakistani government says up to 20 million people have now been affected by the monsoon floods and at least 1,500 are known to have lost their lives.

"This has been a heart-wrenching day for me and for my delegation," Mr Ban said at a press conference, stood alongside President Asif Ali Zardari.

Ban Ki-moon and President Zardari flew by helicopter over four districts of Punjab, the province known as this country's bread basket.

Now from the air it looks like a land of sprawling lakes. Valuable crops like sugar cane and wheat are under water. Mud houses are submerged, millions have fled.

The visitors were greeted with loud applause when they visited a relief camp set up by the Pakistan military. It is now home to about 1,000 displaced people.

Mr Ban announced a further US$10m from the UN's central emergency response fund, making a total of US$27m from the fund so far, and repeated his calls for the international community to come to Pakistan's aid.

"The people of Pakistan need food, emergency shelters, medicines, clean water," he said.

"We are all deeply concerned about the spread of diarrhoea and other water-borne diseases. All our combined medical capacity will be needed to provide the right drugs and care."