ISLAMABAD-- Floods have isolated about 800,000 people in Pakistan who are now only reachable by air and aid workers need at least 40 more helicopters to ferry lifesaving aid to the increasingly desperate people, the United Nations said.

The appeal Tuesday was an indication of the massive problems facing the relief effort in Pakistan more than three weeks after the floods hit the country, affecting more than 17 million people and raising concerns about possible social unrest and political instability.

"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, but that those tides would begin changing Wednesday. He said the Indus would reach peak flood stage late this week.

"The flood situation is not yet over," Chaudhry said.

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 U.N. 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.

Dr. Jahanzeb Orakzai, Pakistan's national health coordinator, said a team has been formed to oversee the response to any flood-related health emergencies.

"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.

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.

On Monday, President Asif Ali Zardari defended the government's much-criticized response to the 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 Islamabad.

On Tuesday, officials announced that the government would give $230 to every family affected by the floods, with a statement from Zardari's spokesman calling the payment "initial assistance."

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.