Updated at 12:43 p.m. ET
Recovering from the devastating floods still battering Pakistan will take at least three years, the president said, as the waters swept south after leaving millions homeless and submerging millions of acres of farmland.
The floods that began nearly a month ago with hammering monsoon rains in the northwest have affected more than 17 million people, the United Nations estimates. Most of the 1,500 deaths occurred early in the flooding, but the crisis still is growing.
The U.N. also said 800,000 flood victims are only reachable by air and appealed for at least 40 more heavy-lift helicopters to ferry lifesaving aid.
President Asif Ali Zardari defended the government's much-criticized response to the unprecedented floods but acknowledged recovery would take a very long time.
"Three years is a minimum," Zardari said in an interview Monday with a small group of foreign reporters in the capital, Islamabad.
The widespread misery caused by the floods has triggered worries about social unrest, food riots or even a challenge to the government's rule before its term ends in 2013.
Overwhelmed rescuers unable to help entire families are forced to make wrenching decisions - children are torn from their parents' arms, reports CBS News correspondent Mandy Clark.
The U.N.'s appeal was an indication of the massive problems facing the relief effort in Pakistan.
"These unprecedented floods pose unprecedented logistical challenges, and this requires an extraordinary effort by the international community," said John Holmes, U.N. under-secretary-general for humanitarian affairs.
Earlier, Prime Minister Yousuf Raza Gilani said hundreds of health facilities had been damaged and tens of thousands of medical workers displaced and the country's chief meteorologist warned that it would be two weeks until the Indus River the focus of the flooding still sweeping through the country returns to normal levels.
Qamar-uz-Zaman Chaudhry said high tides in the Arabian Sea would slow the drainage of the Indus into it. Those tides, he said, will begin changing on Aug. 25.
"The flood situation is not yet over," Chaudhry said, adding that the Indus would reach peak flood stage late this week.
The floods began with hammering monsoon rains in the northwest and have swept southwards.
Many of those cut off are in the mountainous northwest, where roads and bridges have been swept away.
The United States has deployed at least 18 helicopters that are flying regular relief missions, but the United Nations said it would need at least 40 more heavy-lift choppers working at full-capacity to reach the estimated 800,000 stranded in the country.
The United Nations High Commissioner for Refugees estimates that about 700,000 Pakistanis have been forced into makeshift settlements just in the southern province of Sindh.
While there have been no major disease outbreaks because of the floods, aid agencies are increasingly worried, saying contaminated water and a lack of proper sanitation were already causing a spike in medical problems in camps for the displaced.
"Pakistan and its people are experiencing the worst natural calamity of its history," Gilani said at a meeting on health issues in the flood zone. "As human misery continues to mount, we are seriously concerned with spread of epidemic diseases."
More than 3.5 million children are at risk from waterborne diseases, he said, and skin diseases, respiratory infections and malnutrition are spreading in flooded areas.
The problem is compounded by the flood's impact on the country's medical system which has long been badly overstretched and underfunded. Gilani said the floods had damaged more than 200 health facilities, and that about one-third of the country's 100,000 women health workers have been displaced. Those health workers are the main primary medical care to millions of rural Pakistani women.
Local charities, the Pakistani army and international agencies are providing food, water, medicine and shelter to the displaced, but millions have received little or no help. Aid officials warn that widespread outbreaks of waterborne diseases such as cholera now pose a threat.
Dr. Jahanzeb Orakzai, Pakistan's national health coordinator, said a team was formed Tuesday to oversee the response to any health emergencies arising from the floods.
International medical organizations, including the World Health Organization and UNICEF, will be included in the team, he said.
The medical situation is under control in the flood zone, he said, despite some disease outbreaks, but the situation was still tenuous.
"Health problems usually arise in flood-affected areas after four to six weeks, and we need to be alert and prepared to tackle the situation," he said.
The aid group World Vision said it could be three months until some families are able to return to their homes.
"People are in urgent need of almost everything: shelter, health clinics, clean water, sanitation and livelihood support," Mike Bailey, the group's regional manager for advocacy said in a statement.
On Tuesday, officials announced that the government would give 20,000 rupees ($230) to every family affected by the floods, with a statement from Zardari's spokesman calling the payment "initial assistance."
In Shadad Kot, in the southern province of Sindh, authorities are increasingly worried that even the 11 miles of new levees soldiers have built may not hold back floods in the city, and in Qambar city further to the south.
On Tuesday, workers piled stones and sandbags to plug leaks in the levees, trying to stay ahead of any damage to the defenses. Ninety percent of Shadad Kot's 350,000 residents have already fled the city.
Since the floods first swept the country, the Taliban and al Qaeda have been relatively quiet. But on Monday, three bomb attacks rocked the northwest, one of which killed the head of an anti-Taliban militia on the outskirts of the main city of Peshawar.
Zardari - whose wife Benazir Bhutto was assassinated by militants in 2007 - said the flood crisis would not distract from the campaign against militants. He reiterated earlier remarks that extremists could benefit by picking up recruits from hard-hit areas.
"The fight goes on, on all fronts," Zardari said in the presidential palace with his foreign minister and key advisers sitting alongside him. "If you are fighting for a cause and the ... situation turns difficult, you don't give up. You bring new resolve to the table."
The floodwaters, which have devastated lives from the mountainous north to the southern plains, are expected to begin draining into the Arabian Sea in the coming days.