France must expect "new attacks" by terrorists, with more "innocent victims," the French prime minister Manuel Valls warned yesterday when he spoke on Europe 1 radio. He also revealed that French police are monitoring 15,000 people who are "in a process of radicalisation."

Valls told the radio station: "every day, the intelligence services, the police, the national gendarmerie, thwart attacks, and track down terrorists. We are a target, everyone understands this. This week, at least two attacks have been foiled."

More information about one of those planned attacks has now emerged. Several women and a 15-year-old boy have been arrested in connection with a failed terror attack on Notre Dame cathedral in Paris. The Guardian reports that one woman has been charged.

Valls went on to give detailed numbers of the scale of the problem in France:

We have around 700 jihadists fighting in Iraq and Syria. Among them, 275 women and dozens of children. 196 jihadists have died in Iraq or Syria ... In our country, 1,350 people are under investigation, who are in prison, 293 are directly linked with terrorist networks. We can say that there are 15,000 people who are followed because they are in a process of radicalisation.

The newly-revealed total of 15,000 being monitored by the authorities is even larger than the previously-estimated figure of 11,000, which the Washington Post suggested would need 220,000 officers in order to provide full surveillance.

By contrast, the US newspaper believed that there were around 3,000 people being watched in the UK. Although considerably smaller than the number in France, this would still require half of the police officers in England and Wales in order to monitor them properly.

Yesterday's comments by Valls indicate the scale of the problem faced by security forces in Europe. It also underlines the need for alternative approaches to dealing with possible threats other than assigning an unsustainably high proportion of a country's security forces to watch people.