UN Secretary General Ban Ki-moon has described as "heart-wrenching" the destruction he witnessed on a visit to flood-devastated Pakistan.
Mr Ban said the scale of the disaster was greater than anything he had seen before.
He again urged the world to speed up aid to the country, saying shelter and medicine were desperately needed.
The Pakistani government says up to 20 million people have now been affected by the monsoon floods.
At least 1,500 are known to have lost their lives.
Health experts are warning that the threat of epidemics in flood-hit areas is growing.
"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.
"I will never forget the destruction and suffering I have witnessed today.
"In the past I have seen scenes of natural disaster around the world, but nothing like this. The scale of this disaster is so large. So many people in so many places in so much need."
He announced a further $10m (£6.4m) from the UN's central emergency response fund, making a total of $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."
He said one fifth of Pakistan had been ravaged by floods.
"The flood waves must be matched with waves of global support," he said.
The flooding began more than two weeks ago in the mountainous north-west of Pakistan and has swept south across a quarter of the country including its agricultural heartland.
The International Monetary Fund has warned that the floods could have dire long-term economic consequences for a country already reliant on foreign aid.
On Wednesday the UN launched a $459m appeal for emergency aid for Pakistan. It said that billions of dollars would be needed in the long term.
The US has already donated at least $70m to the country, which is a key regional ally in fighting terrorism.
During his visit, Mr Ban held talks with Prime Minister Yusuf Raza Gilani and President Zardari, whose handling of the crisis has been widely criticised.
In his first public comments since returning from a much criticised visit to Europe, Mr Zardari defended the official response.
"The government has responded very responsibly," he said, adding that the army, the police and officials were all working to relieve the suffering.
"I would appeal to the press to understand the magnitude of the disaster."
Mr Zardari described the floods as "our time of endurance, our time of need".
"It is a time when the nation will stand together," he said.
"I call upon the whole of the nation, all of Pakistan, indeed I call upon the world to support us and to listen to the voice of the United Nations to support Pakistan."