South Pakistan was braced for yet more flooding on Thursday as water surged down the Indus river from the north.
Record monsoon rains and melting glaciers in the country's northern mountains have left a third of the country under water.
The floods are thought to have killed at least 1,200 people, including more than 400 children, according to the National Disaster Management Authority.
The disaster has been blamed on climate change.
“This is not a conspiracy, this is a reality and we need to be mindful,” said foreign ministry spokesman Asim Iftikhar.
Pakistan's meteorological office predicted more rains and flash flooding for September.
"Overall, a tendency for normal-to-above-normal precipitation is likely over the country during September," the office said in monthly outlook posted on its website on Thursday.
North-eastern Punjab province and the southern Sindh province are expected to receive above-normal rainfall, it said. The office said isolated heavy downpours could trigger flash flooding.
Planes carrying supplies were flying into Pakistan on Friday to assist families and children at special risk of disease and homelessness.
The ninth flight from the UAE and the first from Uzbekistan were the latest to land in Islamabad overnight as a military-backed rescue operation elsewhere in the country reached more of the three million people affected by the disaster.
Two more planes carrying aid from the UAE and Qatar were to arrive later on Friday. A Turkish train carrying relief goods for flood victims was on its way to Pakistan, according to the foreign ministry.
Pakistan floods: dramatic moment as rescuer saves cat from flooded stream
The military said on Thursday that it had evacuated about 50,000 people, including 1,000 by air, since rescue efforts began.
The UN has appealed for $160 million to help with what it has called an "unprecedented climate catastrophe". Britain pledged $17 million in aid on Thursday.
"We are on a high alert as water arriving downstream from northern flooding is expected to enter the province over the next few days," Sindh provincial government spokesman Murtaza Wahab told Reuters.
Mr Wahab said a flow of 17,000 cubic metres per second was expected to swell the Indus, testing its flood defences.
Pakistan received about 190 per cent more rain than its 30-year average in the quarter from June to August, totalling 390.7 millimetres.
Sindh, with a population of 50 million, has been the hardest-hit province, receiving 466 per cent more rain than the 30-year average.
Some parts of the province resemble an inland sea, with only occasional patches of trees or raised roads breaking the surface of murky flood waters.
'No one has come to help us'
Hundreds of families have taken refuge on roads, the only dry land in sight for many. Villagers along a road near the town of Dadu were begging journalists for food and other help on Thursday, Reuters reported.
Many are headed for urban centres, such as the port city of Karachi, which has, for now, escaped the flooding.
"We lost our house to the rain and floods. We are going to Karachi to our relatives. No one has come to help us," said Allah-Bakash, 50, leaving with his family and belongings loaded on a lorry.
The floods have swept away homes, businesses, infrastructure and roads. Crops have been destroyed and 810,000 hectares of farm land inundated.
The government said 33 million of the country's 220 million people have been affected.
The National Disaster Management Authority said more than 480,000 people had been displaced and were being looked after in camps but even those not forced from their homes face peril.
The World Health Organisation said more than 6.4 million people were in dire need of humanitarian aid.
