Up to 3.5 million children are at high risk from deadly water-borne diseases in Pakistan following the country's floods, a UN spokesman has said.
In southern Pakistan, floods continue to cause havoc with water surging from the province of Sindh to neighbouring Balochistan.
UN Secretary General Ban Ki-moon, who visited Pakistan, said the floods were the worst disaster he had seen.
However, the UN has so far only raised a fraction of the aid it has asked for.
"Up to 3.5 million children are at high risk of deadly water-borne diseases, such as watery diarrhoea and dysentery," Maurizio Giuliano, spokesman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA), is quoted as saying by the AFP news agency.
"What concerns us the most is water and health. Clean water is essential to prevent deadly water-borne diseases. Water during the flood has been contaminated badly. There is a shortage of clean water," he added.
The World Health Organization was also preparing to assist tens of thousands of people in case of cholera, although the government has not notified the UN of any confirmed cases, he added.
He estimated the number at risk from such diseases was six million.
Pakistani Foreign Minister Shah Mehmood Qureshi told the BBC that he feared the growing desperation of flood victims could play into the hands of extremists.
But he said troops fighting insurgents in the north had not been redeployed to help the relief effort.
"We have moved additional troops to southern parts of Punjab and the provinces of Sindh and Balochistan. We are not going to permit militants to take advantage of this situation," he said.
In southern Pakistan, angry flood survivors blocked a main road in Sindh province to protest against the slow delivery of aid and demanded more action from the authorities.
One of the protesters, Mohammad Laiq, said the government had to do more to help people.
"There seems to be no government here since the floods. We lost our children, our livestock, we could hardly save ourselves - though we have come here but we are getting nothing.
"Where is the government? What do we do? Where do we go? We have to tell the government and it is the responsibility of the government to do whatever is possible," he said.
Saleem Bokhari, whose village in the Layyah District of Punjab is under water, told the BBC that the situation was worsening moment by moment.
"Due to standing water there is a rapid production of mosquitos, abdominal disease, fever, malaria and skin diseases," he said.
"Government officials and volunteers are only reaching the cities. Villages or remote areas are helpless."
Sindh Irrigation Minister Jam Saifullah Dharejo said water levels were still rising and now entering the Shikarpur district.
"The next five days are crucial as more water is going to pass through the river towards Thatta," he said.
In eastern Balochistan, at least one district centre and three major towns have been inundated following a government decision to divert the thrust of the flood in the Indus river away from Jacobabad, a major town in the north-west of Sindh province, and the nearby Pakistan Air Force base.
