UN Secretary-General Ban Ki-moon has said he has never seen anything like the flood disaster in Pakistan after surveying the devastation and urged foreign donors to speed up assistance to the 20 million people affected.
Mr Ban called on foreign donors to speed up aid to Pakistan after the country's worst floods in decades disrupted the lives of more than a tenth of the country's 170 million population.
Pakistan's mountain valleys and fertile plains have been flooded, killing up to 1,600 people and leaving two million homeless.
Six million people still need food, shelter and water and medicine, the United Nations says.
"This has been a heart-wrenching day for me," Mr Ban said after flying over the hard-hit areas with President Asif Ali Zardari.
"I will never forget the destruction and suffering I have witnessed today. In the past I have witnessed many natural disasters around the world, but nothing like this," he added.
But with an area roughly the size of Italy hit by floods, government and foreign aid has been slow in coming and the United Nations has warned of a second wave of deaths among the sick and hungry if help does not arrive.
"Waves of flood must be met with waves of support from the world," said Mr Ban. "I'm here to urge the world to step up assistance," he said.
President Zardari has been criticised for his response to the disaster, especially for going ahead with a state visit to Europe just as the crisis was unfolding. Mr Zardari has visited victims twice since returning, but images of him at a family owned chateau while in France are likely to hurt him for months to come.
In his first comments to the media since returning, he defended the government.
"The government has responded very responsibly," he said, saying the army, the police, the navy and officials were all working to relieve the suffering. "I would appeal to the press to understand the magnitude of the disaster."
Mr Zardari said it would take up to two years for the country to recover.
The UN has reported the first case of cholera amid fears that disease outbreaks could spread with survivors sleeping in makeshift tarpaulin tents. Some beg or loot.
Bridges have collapsed, highways have been snapped in two by torrential rains and villages have been cut off from the outside world in what was already one of the poorest countries in Asia.
Only a quarter of the $459 million aid needed for initial relief has arrived, according to the United Nations.
"I am here ... to share my sympathy and solidarity of the United Nations together with the people and government of Pakistan at this time of trial," Mr Ban said on arriving in Pakistan.
"I am here also to urge the world community to speed up their assistance to Pakistan."
Mr Ban's visit comes as millions of Pakistanis are increasingly frustrated by the government that has already been hit by political bickering and Taliban militant violence.
Pakistan's government has been accused of being too slow to respond to the crisis with victims relying mostly on the military - the most powerful force in Pakistan - and foreign aid agencies for help.
Floodwaters pose new threats to the populous Sindh province and the southwest province of Baluchistan, a region also hit by a decades long separatist insurgency.
At least 500,000 tonnes of wheat have been destroyed by the floods. At Kot Addu in southern Punjab, thousands of bags lay ruined as workers were unable to move them quickly enough from rising floodwater.
Highlighting the lack of logistical support and helicopters for relief efforts, flour, cooking oil and rice were carried by mules along narrow mountain tracks to 150,000 people in Shahpur in the northwest Swat valley.
Despite the government's perceived failure to tackle the crisis, a military coup is unlikely. The army's priority is fighting Taliban insurgents, and seizing power during a disaster would make no sense, analysts say.
Opposition leader Nawaz Sharif and Gilani have said they would leave politics aside in the crisis, possibly helping to create more political stability.
The International Monetary Fund has warned of major economic harm and the Finance Ministry said it would miss this year's 4.5 per cent gross domestic product growth target.
Any economic downturn would come just as the government aims to fund projects to win hearts and minds in the battle against the Taliban.
Wheat, cotton and sugar crops have all suffered damage in a country where agriculture is a mainstay of the economy.
Waters roared down from the northwest to Punjab province to Sindh, where more flooding is expected. Sindh is home to Pakistan's biggest city and commercial hub Karachi. Floods have damaged mostly rural areas there, although concerns are rising that other urban centres are at risk.
Reuters