Rescue teams in northern Pakistan are battling to reach tens of thousands of people cut off by monsoon flooding.
While water is receding in some areas, many communities remain cut off by the region's worst flooding for 80 years.
The UN said 3m people had been affected and more than 1,400 had been killed. The government said some 27,000 people remained trapped and awaiting help.
Renewed rain on Tuesday slowed the relief effort, with criticism rising of the pace of the government response.
The United Nations' World Food Programme says it has provided emergency food for 42,000 people in Pakistan by Monday and that by the end of the week it expects to have helped 250,000 people.
However, WFP spokeswoman Emilia Casella said about 1.8 million needed food aid.
On Tuesday, the White House issued a statement vouching support for Pakistan during the crisis:
"Our relationship with Pakistan goes far beyond our shared commitment to fight extremists. The United States government stands ready to continue to assist Pakistani authorities address the difficult challenges posed by this natural disaster," it said.
In the Swat Valley, where reconstruction was under way after a major military operation against the Taliban during 2009, the flooding has brought down bridges and left communities cut off.
The Malakand region, which includes Swat, is among the worst-affected areas, with roads and bridges washed away.
Local official Mian Iftikhar Hussain said rescue teams were trying to reach 27,000 stranded people, including 1,500 tourists in the Swat Valley.
Adnan Khan, a relief official in the worst-affected province, Khyber-Pakhtoonkhwa, told Reuters news agency: "The entire infrastructure we built in the last 50 years has been destroyed."
The BBC's Lyse Doucet in Islamabad says the flooding would be a challenge in any country, but is a growing crisis for a country like Pakistan, already suffering economic woes and a Taliban insurgency.
The return of monsoon rains has grounded helicopters and raised fears of renewed flooding.
Forecasters said rain would continue in the north-west and in southern provinces of Punjab and Sindh over the coming days.
The Pakistani military says it has committed 30,000 troops and dozens of helicopters to the relief effort, but winching individuals to safety is a slow process.
Some survivors have complained that the government has responded slowly; several hundred people protested in the city of Peshawar, where homeless survivors have crammed into temporary shelters, and there have been angry scenes in Swat.
In Nowshera district one flood victim, Faisal Islam, told the Associated Press: "We need tents... This is the only shirt I have. Everything else is buried."
Aid agencies say the risk of water-borne diseases spreading will remain high until the floodwaters fully recede. Mr Hussain, the regional information minister, said there were reports of cholera emerging in the Swat Valley.
The UN children's agency Unicef said more than a million children needed emergency aid.
Governments around the world have pledged millions of dollars in aid, but there has been no decision yet on whether to launch a global appeal for aid.