Floodwaters have already damaged the northwest, leaving more than 3 million people to deal with destroyed houses and crops, and a lack of food and water.
People are being helped by army to evacuate from the central Punjab and southern Sindh provinces. Monsoon rains are likely to continue.
United Nations Secretary-General Ban Ki-moon has dispatched his special envoy, Jean-Maurice Ripert, to Pakistan to join the U.N. team already occupied in relief efforts.
At least 1,500 people have lost their lives in the floods. Pakistani Prime Minister Yousuf Raza Gilani said so far 100,000 people have been rescued.
The World Food Program said almost 2 million people are in need of food assistance.
In Washington, U.S. Secretary of State Hillary Clinton said Wednesday that Pakistan can count on long-term U.S. support. She outlined U.S. aid that has already been sent and announced a program in which Americans can contribute to the relief effort.
The International Red Cross said that in addition to other damage, floodwaters have destroyed much of the health infrastructure in the worst affected areas, leaving people especially vulnerable to water-borne diseases.
The United Nations, Britain and Canada also have committed millions of dollars in aid to Pakistan.
Islamist charities, some with suspected ties to militant groups, also have stepped in to provide aid to flood victims.
Prime Minister Gilani announced Wednesday that Cabinet ministers will contribute one month's salary to flood victims.