Pakistan’s prime minister Shehbaz Sharif has warned of a “humongous challenge” ahead following catastrophic floods in the country that have coincided with an acute financial crisis and a deepening political conflict between the government and former leader Imran Khan.

The disaster during a monsoon season that came after a spring of baking temperatures has made Pakistan a case study among countries vulnerable to climate change that are also battling interlinked humanitarian, economic and political crises.

After flood waters swept away settlements, pooled in low-lying areas and caused the country’s largest lake to overflow on Wednesday, officials said about 1,400 people had been killed and as many as 40mn, or about a fifth of the population, had been displaced. A third of the country is under water and more than half of its 160 districts have been declared “calamity hit”.