The United Nations chief said Wednesday it was time for "meaningful action" on the issue of compensation for damage wrought by the climate crisis, especially in developing countries like Pakistan.

Ahead of the forthcoming COP27 UN climate summit in Egypt, Secretary-General Antonio Guterres and Egyptian President Abdel Fattah al-Sissi -- the latter appearing by video link -- co-hosted a meeting of world leaders for "frank exchanges" on climate action.

"My messages were stark," Guterres told reporters at the UN General Assembly following the meeting. "On the climate emergency: The 1.5-degrees limit is on life support –- and it is fading fast," he said, referring to the Paris accord goal of limiting long-term warming to 1.5C above pre-industrial levels.

"You have all seen the appalling images from (flooding in) Pakistan. This is happening at just 1.2 degrees of global warming, and we are headed for over three degrees."

He called on governments to tackle "four burning issues" between now and COP27: greater ambition to keep the 1.5C possible; meeting financial commitments to the developing world; increasing support for adaptation measures, and the issue of "loss and damage."

This last point has become a critical area in climate negotiations. It concerns damage already caused by multiplying extreme weather events, which neither the measures to mitigate global warming nor those to adapt to its impacts have been able to prevent.

Developing nations argue that historic polluters have a moral imperative to pay for the loss and damage, but the idea was shot down by rich nations at COP26, who offered only to start talking about the issue at COP27.

A few days ago, the group of least developed countries meeting in Dakar once more pushed on the issue, calling for the establishment of a "funding mechanism" to deal with the damage caused by global warming.

"I hope COP27 in Egypt will take it up, as a matter of climate justice, international solidarity and building trust," said Guterres.

Disease, malnutrition threaten to raise Pakistan flood toll

The water-borne diseases and malnutrition that are plaguing swathes of Pakistan after record monsoon floods threaten to be more deadly than the initial deluge, UN officials warned.

Pakistan has been lashed by unprecedented monsoon downpours flooding a third of the country -- an area the size of the United Kingdom -- and killing nearly 1,600 people, according to the latest government figures.

More than seven million people have been displaced, many living in makeshift tents without protection from mosquitoes, and often with little access to clean drinking water or washing facilities.

United Nations humanitarian coordinator Julien Harneis said Pakistan faced a cascading "second disaster" from diseases such as dengue, malaria, cholera and diarrhoea, as well as malnutrition.

"My personal concern is that mortality from the water-borne diseases, from malnutrition will be higher than what we've seen so far," he told a press conference in Islamabad on Wednesday.

"That's a sober but realistic understanding."

Around 33 million people have been affected by the floods, which have destroyed around two million homes and business premises, washed away 7,000 kilometres (4,300 miles) of roads and collapsed 500 bridges.

- 'Worried about thousands' -

Swathes of farm land -- mostly in the southern province of Sindh -- still remain under water.

Dengue cases there have soared to more than 6,000 since the start of the year -- half in September alone -- and are already approaching the total figures for 2021.

But the devastation is so widespread and ongoing -- with some communities still cut off -- that a full picture of the tragedy has yet to emerge.

"Five hundred children died because of the direct impact of the floods," said UNICEF field operations chief Scott Whoolery.

"We're not worried about hundreds. We're worried about thousands," he said of the health crisis.

"Many of them we probably will never know, they won't be counted."

The UN has already received pledges surpassing its initial campaign to raise $160 million for flood relief, but now plans to up that goal.

"The absolute priority is to deal with the health crisis that is striking the flood-affected districts right now," said Harneis.

Pakistan was already ravaged by extreme weather once before this year, with extreme heat waves scorching the country in spring.

Scientists have linked both events to human-caused climate change.

The South Asian nation -- home to more than 220 million -- is responsible for less than one percent of global greenhouse gas emissions.

But it ranks eighth on a list compiled by the NGO Germanwatch of countries most vulnerable to extreme weather caused by climate change.