PARIS (AFP) - Two brothers suspected of having gunned down 12 people in an Islamist attack on satirical magazine Charlie Hebdo were recognised on Thursday morning in northern France when they reportedly robbed a petrol station, sources close to the manhunt and media reports said.

The suspects stole food and petrol, firing shots as they struck at the roadside stop near Villers-Cotterets, in the Aisne region, French media reported according to BBC.

The manager of the station "recognised the two men suspected of having participated in the attack against Charlie Hebdo", the source close to the manhunt said.

They are said to have driven off in the direction of Paris in a Renault Clio car, apparently the same vehicle hijacked in Paris soon after the Charlie Hebdo attack on Wednesday, BBC said.

According to French commercial channel BFMTV, police are monitoring all of the main entry roads into the capital.

French social media carried numerous reports of police helicopters across northern France. Police tightened security at transport hubs, religious sites, media offices and department stores.

There were scattered, uncomfirmed reports of sightings of the assailants and police increased their presence at entry points to Paris. One police source talked of a type of "psychosis" setting in with various reports and rumours, but police had to take each of them seriously.