PARIS, March 18 (CNA) - France imposed a month-long lockdown on Paris and parts of the north after a faltering vaccine rollout and spread of highly contagious coronavirus variants forced President Emmanuel Macron to shift course.

Since late January, when he defied the calls of scientists and some in his government to lock the country down, Macron has said he would do whatever it took to keep the euro zone's second largest economy as open as possible.

However, this week he ran out of options just as France and other European countries briefly suspended use of the AstraZenca vaccine.

His prime minister, Jean Castex, said France was in the grip of a third wave, with the virulent variant first detected in Britain now accounting for some 75 per cent of cases. Intensive care wards are under severe strain, notably in Paris where the incidence rate surpasses 400 infections in every 100,000 inhabitants.

"The epidemic is getting worse. Our responsibility now is to not let it escape our control," Castex told a news conference.

France reported 35,000 new cases on Thursday and there were more COVID patients in intensive care in Paris than at the peak of the second wave.

Now was the time to tighten restrictions, Castex said.

"Four weeks, the time required for the measures to generate a sufficient impact. (It is) the time we need to reach a threshold in the vaccination of the most vulnerable."

The lockdowns will kick in from Friday at midnight in France' 16 hardest-hit departments that, with the exception of one on the Mediterranean, form a corridor from the northern Channel port city of Calais to the capital.

Barbers, clothing stores and furniture shops will have to close, though bookstores and others selling essential goods can stay open.

Schools will stay open and people will be allowed to exercise outdoors within a 10km radius of their homes. Travel out of the worst-hit areas will not permitted without a compelling reason.