Waters have exceeded the danger level at a key flood barrier in Pakistan's southern province of Sindh.
The Sukkur Barrage flooding means Sindh faces as much devastation as that seen further north in Khyber Pakhtunkhwa and Punjab provinces, say experts.
Enraged survivors have been physically attacking government officials in flood-hit areas, amid widespread anger at the pace of the relief effort.
At least 1,600 people have died in the region's worst deluge in 80 years.
With more than 14 million people already affected, the monsoon rains show little sign of abating.
On Monday, a new international radio initiative called "info-as-aid" made its first broadcasts in Urdu and Pashto in an effort to spread information about accessing aid and also about countering disease.
TV channels 'jammed'
Flood waters have roared down from the north to the agricultural heartland of Punjab and on to Sindh along a trail more than 1,000km (600 miles) long.
In the early hours of Monday morning, the water flow coming down the Sukkur Barrage was recorded at up to 1.4m cusecs (cubic feet per second). It is only designed to withstand 900,000 cusecs.
Upper Sindh is already under water, and rescuers are continuing to evacuate people from the province, where the Indus river banks are at risk of bursting. Two million people have already left the area.
Meanwhile, two major private Pakistani television channels, Geo and Ary, have reportedly been blocked in Karachi and other parts of Sindh.
No official reason has been given, although correspondents say media criticism of President Asif Ali Zardari and his government's response to the flood disaster is likely to have played a key role.
The networks had been reporting how a shoe was thrown at Mr Zardari during a rally organised by his Pakistan People's Party in England on Saturday.
Police in the city of Birmingham said they escorted a heckler from the venue after he hurled the missile, which missed the president.
But the Pakistani government has denied the incident happened.
Flood survivors have bitterly accused the authorities of failing to come to their rescue, with Mr Zardari in particular condemned for his trip to Europe last week.
On Sunday, a senior government minister's convoy was attacked as she visited her constituency in the Muzaffargarh district of southern Punjab.
A policeman was injured as angry locals threw stones at the convoy transporting Hina Rabbani Khar, the minister for economic affairs.
In parts of the Khyber Pakhtunkhwa and Punjab, officials have also received a rough welcome from flood survivors unhappy with the relief effort.
Prime Minister Yousuf Raza Gilani said the country had been set back years, as he visited Sindh on Sunday.
The entire Swat valley, in north-west Pakistan, was cut off at the weekend, with even helicopters unable to reach it because of the poor weather.
With roads, bridges and railway tracks washed away, and deadly landslides increasing the isolation of many of the worst-hit areas, aid workers are having to use donkeys to deliver relief.
"It's hard to get supplies there. I would like to emphasise we are moving by foot or donkey. We are unable to get in to most places of Swat valley," Maurizio Giuliano, spokesman for the UN Office for the Co-ordination of Humanitarian Affairs, told news agency Reuters.
In the far north of the country, dozens of people were killed on Saturday when two villages were buried in mud and rocks. Nearly 30 bodies were recovered from rubble after landslides in Gilgit-Baltistan province.
The UN has said that Pakistan will need billions of dollars in aid to recover.
Meanwhile, charities with links to militants have been delivering aid to thousands of flood victims, as they did during the earthquake that devastated part of Pakistani-administered Kashmir in 2005.
BBC Urdu will transmit six daily bulletins in Urdu and Pashto providing vital information including how to stay safe, avoid disease and access aid.