PARIS (AFP) - Seven people have been detained in the hunt for two brothers suspected of gunning down 12 people in an assault on a satirical weekly, a judicial source said.

Confirming earlier comments by Prime Minister Manuel Valls, the source, who refused to be named, said men and women close to the brothers - who are among three suspects in the attack on Charlie Hebdo - were currently being questioned by police. The source did not say where the seven had been detained.

Police have issued arrest warrants for Cherif Kouachi, 32, a known militant convicted in 2008 for involvement in a network sending fighters to Iraq, and his 34-year-old brother Said. Both were born in Paris.

Le Figaro newspaper said two of those detained were the sister of Cherif Kouachi and her husband, as well as the wife of Said Kouachi.

A third suspect - 18-year-old Hamyd Mourad - had surrendered to police. A spokesman for the Paris prosecutor said that Mourad had walked into a police station in Charleville-Mézières, about 145 miles north-east of Paris, and surrendered, New York Times reported.

"He introduced himself and was put in custody," Agnès Thibault-Lecuivre was quoted as saying. Mourad was reportedly from the north-eastern city of Reims.

The prime minister had told RTL radio that the brothers - who are still on the run - were known to intelligence services and were "no doubt" being followed before Wednesday's attack.

The masked, black-clad gunmen burst into the offices of the magazine on Wednesday morning, killing some of France's most outspoken journalists and two policemen, before jumping into a car and escaping.