From Sunday all but essential travel from outside of the EU will be banned. Testing requirements from inside the EU will also be tightened.
Castex said police would increase their enforcement of an evening curfew and other rules already in place.
Infections remain high in France, despite recent rule tightening.
"We can still give ourselves a chance to avoid confinement," Jean Castex said at the Elysee Palace at the end of a Defence Council meeting.
He warned "the question of a containment is legitimately raised" due to the number of Covid-19 cases the country is dealing with, stressing that "the next few days will be decisive".
As we reported earlier, The Embassy of Ukraine in France has addressed the Paris Prosecutor's Office with an official request regarding the brutal beating of a Ukrainian teenager.
"The diplomatic mission has made an official request to the Paris Prosecutor's Office, it cooperates with law enforcement agencies in order to conduct a prompt and objective investigation. The consul is in contact with relatives of the teenager," Oleh Nikolenko, Spokesman for the Foreign Ministry of Ukraine,said.