The World Health Organization (WHO) has urged countries to return to "basic principles" of public health surveillance in the fight against COVID-19.

In a briefing on Friday, the WHO, which said it is facing a $1.3 billion funding deficit for its effort to tackle COVID-19, issued the call for more surveillance as many countries have embarked on reopening their economies.

Mike Ryan, head of the WHO's health emergencies programme, urged all countries to focus on the fundamentals of the global coronavirus fight: scouting potential new infections, hunting them down, confirming them and then separating those afflicted, to save others from the disease.

"We seem...to be avoiding the uncomfortable reality that we need to get back to public health surveillance," said Ryan. "We need to go back to where we should have been months ago — finding cases, tracking cases, testing cases, isolating people who are tested positive, doing quarantine for contacts."

The call comes as the number of COVID-19 infections globally surpassed 3.9 million, with the deaths going beyond 272,000.

In Friday's briefing, Ryan also urged nations to stick together as the disease spreads from country to country, sometimes at different rates and with wide swings in death tolls.