UN Secretary General Antonio Guterres has called a flood that killed thousands in Derna, Libya a symbol of the world's ills as he opened the annual General Assembly on Tuesday.

"Even as we speak now, bodies are washing ashore from the same Mediterranean Sea where billionaires sunbathe on their super yachts," Guterres said.

"Derna is a sad snapshot of the state of our world – the flood of inequity, of injustice, of inability to confront the challenges in our midst."