Charlie Hebdo Paris massacre: Killers are military trained Islamic militants

Terrorism experts believe the shooting that killed 12 people at satirical magazine Charlie Hebdo was carried out by professional "highly trained" Islamic militants.

A major manhunt is currently underway to track the three gunmen who shot dead 10 magazine staff including editor-in-chief Stéphane Charbonnier as they stormed the magazine's headquarters in Paris shouting "Allahu Akbar" - God is greatest - in a revenge attack against the publication.

Two police officers are also among the dead.

Experts at Northeastern University in the US believe the attackers, who were armed with Kalashnikov rifles and are also thought to have had a rocket-propelled grenade were "highly trained" individuals who had meticulously planned the military operation.

"There were multiple attackers involved and they seem to be quite professional, unfortunately," Max Abrahams told IBTimes US.

"Even though Charlie Hebdo is a soft target, it's not that soft. It has a police presence, there was a code at the building and members of the publication were vigilant because they knew they were under threat, yet the assailants seem to have had no problem going in there killing staff, two cops and then successfully fleeing the scene."

He added: "These seem to have been Islamist perpetrators who are highly trained and skilled. The dispersion of the bullets was very accurate, suggesting they were a good shot."

Abrahams now is concerned the group could strike again or be returning Islamic State jihadists – encouraged by Abu Bakr al-Baghdadi, the leader of Islamic State.

"I'm worried with the sophistication of this early plan that it could develop in a similar fashion," he said. "If you look at the Norwegian attacker Breivik and the Mumbai attackers, for example, they all had had multiple scenarios planned, so this may not be a one-off attack.

"The biggest fear as we all know is that Westerners will link up with jihadist groups like IS in Syria and then return home radicalized with enhanced tactical skill."

Richard Clarke, former White House counter-terrorism advisor, said that the gunmen "carried out the operation in a very calm, controlled way", in an ABC interview.

"They were people who did not look like they were wild, on some kind of spree, but who were accomplishing a military operation," he said. "They are moving quickly and appear to be physically fit. This doesn't equal a group of rogue, angry guys."

The assault has been labelled France's worst terrorist attack to hit the country in 50 years with President Francois Hollande describing it as "an act of exceptional barbarism".

French authorities have been on high terror alert following a spate of attacks that sparked fears of Islamic terrorism in the lead up to Christmas.

Police said the attackers dumped a black Citroen hatchback before fleeing north and hijacked a Renault.

In 2011, a highly controversial edition of the magazine appeared in November 2011 poking fun at sharia law and the Prophet Mohammed. Soon after their offices were firebombed.