French police have released three female suspects after questioning, but will keep nine other people in custody as part of an anti-terror investigation connected to last week's attacks in Paris that have put Europe on high alert, officials said on Sunday.

In Paris, prosecutor's office spokesman Denis Fauriat said nine suspects among a total 12 rounded up on Friday were facing extended interrogations for the next two days.

Police in at least four European countries have arrested dozens of suspects in recent days in an anti-terrorism crackdown sparked by last week's bloody rampage in and around Paris. Brothers Said and Cherif Kouachi and their friend, Amedy Coulibaly, killed 17 people at the satirical newspaper Charlie Hebdo, a kosher grocery and elsewhere.