Millions of Pakistanis desperately need help 100 days after monsoon rains triggered major floods for a second year running with a "grave risk" of a public health crisis, aid groups warned Wednesday.

The Pakistan Humanitarian Forum (PHF), a network of the 41 largest international charities in the nuclear-armed Muslim country, said more than five million people were affected by the floods -- about half of them children.

Around 700,000 people remain displaced, at least six million acres were ravaged and 2.3 million acres of crops lost in an emergency that received little global attention, PHF said in a new report.

"One hundred days after the 2011 floods began, millions of men, women and children urgently need vital assistance including clean water and sanitation, healthcare, food, shelter, and cash," it said.

Yet a UN appeal for $357 million launched on September 18 has so far attracted just 37 percent of funds, it said.

The report also accused Pakistan of delaying international charities' response to the crisis, by restricting the kind of aid that could be provided and through slow visa processing for foreign aid workers.

"As a result, an under-resourced relief effort has left several million people with little or no aid. Disease and illness are on the rise and, with winter about to begin, there is a grave risk of a public health crisis," it said.

The grouping said chances had been missed to help thousands of farmers plant winter crops, increasing the risk of hunger in a region where food shortages and malnutrition are already at emergency levels.

Pakistan's National Disaster Management Authority was not immediately available for comment, but one government official contested the complaints.

"How we can stop anyone from helping us, our people, in the crisis?" he told AFP on condition of anonymity.

"There could be a couple of odd cases as visa issuance could take some time, but it is not our policy."

Pakistan this week topped a league table for countries worst hit by extreme weather events, according to a "climate risk index" published in South Africa.

Pakistan was hit by the worst floods in its history in 2010.

"Fundamentally, Pakistan must stop staggering from one crisis to another," PHF said. "Much greater investments are needed to prevent every flood, drought or earthquake from becoming a human disaster," it added.