The UN is to launch an appeal to help Pakistan tackle the country's worst flooding in 80 years.
UN Secretary General Ban Ki-moon issued an urgent plea for donors to "generously support Pakistan at this difficult time".
A UN official said the disaster had now affected nearly 14 million people - eclipsing the scale of the 2004 Indian Ocean tsunami.
At least 1,600 people are known to have died so far in Pakistan.
"We will soon issue an... appeal for several hundred million dollars to respond to immediate needs," Mr Ban told a news conference.
UN officials and the Pakistani government have appealed for more international help as the monsoon rains show little sign of abating.
The flood waters which devastated the north-west of the country have moved south to the agricultural heartland of Punjab and on to Sindh.
The torrents are battering the key Sukkur Barrage, threatening to further inundate large areas of Sindh.
Upper Sindh is already under water, and two million people have fled the province as the Indus river threatens to burst its banks.
Maurizio Giuliano, a spokesman for the UN Office for the Co-ordination of Humanitarian Affairs, told the BBC that the disaster was affecting 13.9 million people across Pakistan - "more than the world's three disasters combined".
More than three million people were affected by the 2005 Pakistan earthquake, another five million by the 2004 tsunami and three million by the Haiti earthquake in January.
The UN estimates that 1,600 people have died in Pakistan's floods and the Pakistani government has confirmed 1,243 deaths. About 220,000 were killed in the December 2004 tsunami in Asia.