Record monsoon rains and glacier melt in northern mountains have triggered floods that have killed more than 1,391 people, sweeping away houses, roads, railway tracks, bridges, livestock and crops.
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 have been disrupted. Pakistan estimates the damage at US$30 billion, and both the government and Guterres have blamed the flooding on climate change.
The UN secretary-general landed in Sindh province on Saturday, before flying over some of the worst-affected areas en route to Balochistan, another badly hit province.
"It is difficult not to feel deeply moved to hear such detailed descriptions of tragedy," Guterres said after landing in Sindh, according to a video released by the office of Prime Minister Shehbaz Sharif.
"Pakistan needs massive financial support. This is not a matter of generosity, it is a matter of justice."
A video released by Information Minister Marriyum Aurangzeb showed Guterres seated next to Sharif viewing flood-damaged areas from an aircraft window. "Unimaginable," Guterres said, surveying the damage.
In July and August, Pakistan got 391mm of rain, or nearly 190 per cent more than the 30-year average. The southern province of Sindh has seen 466 per cent more rain than average.
Guterres said on Saturday the world needed to understand the impact of climate change on low-income countries.
"Humanity has been waging war on nature and nature strikes back," he said.
"Nature strikes back in Sindh, but it was not Sindh that has made the emissions of greenhouse gases that have accelerated climate change so dramatically," Guterres said. "There is a very unfair situation relative to the level of destruction."