The Pakistani government has been criticised for its response to the flooding.
"The government is not able to move itself to a position where it can help people," Al Jazeera's Kamal Hyder said, reporting from Muzaffargarh in southern Punjab.
As the humanitarian disaster pushed into a second week, Ban Ki-moon, the UN secretary-general dispatched special envoy Jean-Maurice Ripert to help mobilise international support and address the victims' "urgent, immediate needs," a spokesman said.
Ripert, a former French ambassador to the UN, arrived in Pakistan on Thursday and will visit affected areas in the northwest and meet government officials.
The record rains triggered floods and landslides last week that devastated villages and farmland in some of the country's poorest and most volatile regions in northwestern Khyber Pakhtunkhwa and central Punjab provinces.
"We see urgent need of food assistance to people affected by floods to prevent a starvation-like situation," a spokesman for the UN World Food Programme warned.
"Eighty per cent of food reserves have been destroyed by the floods, which also caused massive damage to livestock, markets, roads and overall infrastructure," he said.
The United Nations has also said clean drinking water and sanitation are urgently needed to stop disease spreading among survivors.
New flood warnings
Meanwhile, Pakistan's meteorological department has issued new flood warnings, raising fears that the destruction is not over.
"The flood water is increasing at different points and we are expecting more rain in the next 24 hours," the chief meteorologist for Punjab told the AFP news agency.
"This is an alarming situation. Conditions are miserable in those areas, particularly at Tounsa and Gaddu barrages. There are extremely high floods at Kot Addu and other parts," he said.
Red: worst affected areas
Yellow: moderately affected areas
Striped: Evacuations underway
Survivors complain they have been abandoned by the government, organising protests in the northwestern city of Peshawar and on Wednesday blocking the motorway to Islamabad for 1.5 hours.
Particular scorn has been reserved for Asif Ali Zardari, the country's president, for pressing ahead with visits to Paris and London at the height of the disaster.
"I have spoken to several people, they ... are saying the army seems to be doing a very good job with limited resources, but the government seems to be putting the wrong foot forward every time it tries to take a step," Al Jazeera's Imran Khan said, reporting from Islamabad, the Pakistani capital.
"The government is simply floundering, that is how many people are describing it," our correspondent said.
"This is a fledgling democracy ... [and] the institutions are not geared up to deal with this kind of crisis, however, this is seen as [an] excuse by many people."
'Lack of resources'
Mengesha Kebede, a representative of the United Nations' Refugee Agency (UNHCR), told Al Jazeera that the Pakistani government and international agencies are doing their best, despite the massive crisis.
"None of us are prepared to respond to the needs of 3.2 million people ... the major problem is the lack of resources," he said.
Several foreign countries have stepped in to help, and an international relief campaign has included a promise of a $10m aid package from the United States, while the UN will disburse up to the same amount from an emergency response fund.
"We've sent over boats to help with search and rescue, water purification units to provide clean water for thousands of people and temporary bridges to replace bridges damaged by the floods," Hillary Clinton, the US secretary of state, said.
"This represents just the start of our efforts. We will continue to help Pakistan in the days and weeks ahead," she said.
The British government pledged $8m in aid, while Australia pledged $4.4m and China $1.5m.
Other countries including Indonesia, South Korea and Canada have also promised help.
Khan said that people on the ground are frustrated because the "army seems to be doing a very good job with limited resources, but ... [the] government seems to be putting a foot wrong every time it is trying to take a step".