The UN said on Wednesday that more helicopters were urgently needed to reach people who remain cut off and were at risk of starvation or disease.
"As monsoon floods continue to displace millions in southern Pakistan, an estimated 800,000 people in need across the country are only accessible by air," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
The Pakistani authorities said that while the waters had receded in the hard-hit provinces of Punjab, Khyber Pakhtunkhwa and Pakistan-administered Kashmir, the flood risk remained high in the southern province of Sindh, where the Indus River was threatening to burst its banks.
Flood threats in Sindh
Tens of thousands of people have already been evacuated from vulnerable areas close to one of Sindh's main cities. "Hyderabad and large surrounding districts are still facing a threat," Sindh's irrigation minister Jam Saifullah Dharejo told AFP.
Thousands of irrigation officials have been sent to build up the river barriers in high-risk areas, but the danger persists.
"We are working on a war footing. This is an extraordinary flood and we are at war with the extraordinary floods,” Dharejo said.
Spread of disease
The authorities are also battling to protect the city of Shahdadkot from surging waters. Most of the 100,000 residents in the city have already been moved to safety.
The government is also concerned about the spread of epidemics. On Tuesday, Prime Minister Yousuf Raza Gilani said that more than 3.5 million children were at risk from disease.
One woman, who fell sick with stomach cramps and diarrhea in a camp in Sukkur in Sindh province, has already died.
So far around 1,500 people have been confirmed dead by Pakistani authorities since the floods began. The UN says more than 17 million people are affected, with five million still homeless.
Editor: Anne Thomas