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," 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.
Speaking later at a press conference, Mr Qureshi said Nato officials were in talks with Islamabad on setting up an "air bridge" to fly in relief to cut-off areas.
He said Japan had also sent a team to Islamabad with a view to sending helicopters to help in the relief effort.
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 mosquitoes, 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," 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.
An official at a power station serving parts of both Sindh and Balochistan reported standing in 5ft-deep (1.5m) water inside the station as engineers battled to restore the power supply after breaches occurred in a nearby canal, reports the BBC's Mike Wooldridge in Islamabad.
Call for world help
On Sunday, Mr Ban again urged the world to speed up aid to the country, saying shelter and medicine were desperately needed.
Last week, the UN launched a $459m (£294m) appeal for emergency aid for Pakistan. It said that billions of dollars would be needed in the long-term.
But charities say the response to the UN's appeal has been sluggish.
The US has made the biggest contribution so far, followed by the UK.
Britain's Deputy Prime Minister Nick Clegg has branded the international response as "lamentable".
Officials at the OCHA and the medical charity Medecins Sans Frontieres said on Monday that Pakistan suffered from an "image deficit" with the Western public because of associations with the Taliban and widespread corruption.
The Pakistani government says up to 20 million people have now been affected by the monsoon floods.
At least 1,500 people are known to died.
The flooding began more than two weeks ago in the mountainous north-west 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.
BBC Urdu will transmit six daily bulletins in Urdu and Pashto providing vital information including how to stay safe, avoid disease and access aid. Special programmes will be broadcast each day in Urdu at 12.30, 15.30 and 18.30 and in Pashto at 12.45, 15.45 and 18.45 (local times).