Gunmen have attacked the Paris office of French satirical magazine Charlie Hebdo, killing 12 people and injuring seven in an apparent Islamist attack.

At least three masked attackers opened fire with assault rifles in the office and exchanged shots with police in the street outside before escaping by car.

The gunmen shouted "we have avenged the Prophet Muhammad", witnesses say.

President Francois Hollande said there was no doubt it had been a terrorist attack "of exceptional barbarity".

A major police operation is under way in the Paris area to catch the killers. The number of gunmen was initially reported to be two, but French Interior Minister Bernard Cazeneuve later said they were hunting three "criminals".

Four of the magazine's well-known cartoonists, including its editor-in-chief Stephane Charbonnier were reported among those killed, as well as at least two police officers.

Mr Charbonnier, 47, had received death threats in the past and was living under police protection.

French media have named the three other cartoonists killed in the attack as Cabu, Tignous and Wolinski. Reports say the attack took place during the magazine's daily editorial meeting.

Several of the wounded remain in a critical condition.

The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was fire-bombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.

The latest tweet on Charlie Hebdo's account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.

People had been "murdered in a cowardly manner", President Hollande told reporters at the scene. "We are threatened because we are a country of liberty," he added, appealing for national unity.

US President Barack Obama has condemned the "horrific shooting", offering to provide any assistance needed "to help bring these terrorists to justice".

UK Prime Minister David Cameron said in a tweet: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."