PARIS (AFP) - French authorities have asked their US counterparts to help identify a person who appears to have given orders to one of the militant gunmen during the Paris attacks that left 17 dead, a source said Tuesday.

The mystery person contacted Amedy Coulibaly - who shot dead a policewoman and killed four shoppers in a Jewish supermarket - by e-mail a day after his accomplices Said and Cherif Kouachi gunned down 12 people in January at Charlie Hebdo magazine, the source said.

"If possible find and work with good dudes," the person tells Coulibaly in a message delivered on Jan 8.

"No friends possible, work alone, preference for the first thing planned.

"Go to the easiest and most secure and biggest number to start again several times," the person says, pointing to the possibility of acting in "suburbs if problem in centre."

The e-mailer then provides Coulibaly with the steps to take to use a new email address created just hours before, that belongs to a messaging service managed by a company based in Pennsylvania in the United States, said the source close to the case, who refused to be named.

French investigators notified US authorities on Jan 29, and in June asked them to give them any information they have on the account.

The e-mail exchanges were discovered on Coulibaly's laptop on Jan 9 at the Jewish supermarket, after he was gunned down by security forces.

The gunman and the mystery messager both had the access codes for the same mailbox, which meant they never sent e-mails but put them in the drafts section for the other person to check, before deleting them.

French authorities has been on high alert since the militant attacks.

The country has been named as a priority target by Islamic State in Iraq and Syria (ISIS) militants and several other attacks have been foiled since January.