France's prime minister on Thursday announced face masks will become compulsory throughout Paris, expressing concern over an "undeniable" trend of expanding coronavirus infection in the country.

Jean Castex said 19 more departments have been added to a map with "red" zones of active virus circulation, meaning 21 of France's 94 departments are classified as such. 

Official figures released Wednesday showed more than 5,400 confirmed new cases in just 24 hours, with admissions to hospital and intensive care units on the rise.

There was an "undeniable resurgence" of the Covid-19 epidemic throughout France, Castex told a press conference, with an incidence rate of 39 positive tests per 100,000 of the population -- four times the level of a month ago, and rising among all age groups.

The "positivity rate" -- the percentage of tests that come back positive -- was up from one percent in May to 3.7 percent today, and the so-called "R" rate of viral transmission has risen to 1.4 nationwide, meaning ten infected people are infecting 14 others on average.

More than 800 coronavirus patients are being admitted to hospital every week on average, up from 500 six weeks ago, said the prime minister.

"The epidemic is gaining territory, and now is the time to intervene," he said, appealing to all French people to take infection-prevention measures such as regular hand-washing and mask wearing, and social distancing.

Castex announced that Paris, one of the 21 zones with active virus circulation, will make face masks compulsory throughout the city. He did not give a date.

The government would do everything in its power to avoid issuing new, nationwide stay-at-home orders, the premier added, but the possibility could not be excluded entirely and localised confinements may be on the cards.