U.N. Secretary-General Antonio Guterres appealed to the world for massive help for flood-ravaged Pakistan on Friday as he visited to boost the response to a disaster that the government estimates has caused $30 billion of damage.

Record monsoon rains and glacier melt in northern mountains have triggered floods that have swept away houses, roads, railway tracks, bridges, livestock and crops, and killed more than 1,391 people.

Huge areas of the country are inundated and hundreds of thousands of people have been forced from their homes. The government says the lives of nearly 33 million people have been disrupted. Both the government and Guterres have blamed the flooding on climate change.

"I call on the international community that Pakistan needs massive financial support, as according to initial estimates the losses are around $30 billion", Guterres told a joint news conference in the capital Islamabad, after meeting with Prime Minister Shehbaz Sharif on his two-day visit.

Sharif said "Pakistan needs an infinite amount of funding" for its relief effort, adding the country "will remain in trouble as long as it doesn't receive sufficient international assistance".

Pakistan expects to cut its GDP growth projection for the financial year 2022-2023 to 3% from 5% due to the losses, planning minister Ahsan Iqbal told an earlier news conference.

The United Nations has launched an appeal for $160 million in aid to help Pakistan cope with the disaster.

As well as meeting Sharif and foreign minister Bilawal Bhutto Zardari, Guterres will tour affected areas during his visit.

In July and August, Pakistan got 391 mm (15.4 inches) of rain, or nearly 190% more than the 30-year average. The southern province of Sindh has seen 466% more rain than average.

Guterres said the world needed to understand the impact of climate change on low-income countries.

"It is essential for the international community to realise this, especially the countries who have contributed more to climate change," he said.

The World Health Organization has said more than 6.4 million people need humanitarian support in flooded areas.

The cost of clearing up and rebuilding after the floods has added to concerns about whether the country's can afford to continue paying its debts.

Over the last three weeks the government's bonds have fallen sharply, to almost half their face value in some cases, as international investors have begun to fear a default.

Graphic: Alarm bells ringing for Pakistan's bonds-