The World Health Organization has called for "immediate and concerted action" to protect children from contaminated medicines after a spate of child deaths linked to cough syrups last year.

Cases were exploding in China well before the government's decision to abandon its strict "zero-COVID" policy, the director said, quashing suggestions that the sudden reversal caused a spike.

A week after China began dismantling its tough 'zero-COVID' controls, the World Health Organization warned of "very tough" times ahead and state media reported some seriously ill patients at hospitals in Beijing, raising fears of a wave of infections.

China's financial hub Shanghai said on Sunday it would reopen all schools including kindergartens, primary and middle schools on Sept. 1 after months of COVID-19 closures.