As 1.4 million people move to cities each week, local abilities to keep residents safe can become strained, increasing the risk of disasters, warned the United Nations secretary-general.

"The answer is to build resilience to storms, floods, earthquakes, fires, pandemics and economic crises," said Antonio Guterres in a message for World Cities Day on Wednesday.

Cities are already pressing on with that work, he noted. Bangkok has built underground tanks to store excess rainfall and prevent floods, while Johannesburg inhabitants are joining in efforts to improve public spaces.