A huge manhunt is under way in Paris after three men stormed the offices of satirical magazine Charlie Hebdo armed with a Kalashnikov and a shotgun, killing 12 people.

French anti-terror police forces are raiding a site in the northeastern city of Reims, according to reports.

Liberation newspaper said the police have identified and located the three suspects, citing police sources.

Other media outlets have identified the suspects as brothers, french nationals aged in their early 30s, and an 18-year-old whose nationality is unclear.

One official said the men were linked to a Yemeni terrorist network.

A police union official said the assailants remained at liberty and there were fears of further attacks.

The French government declared the highest state of alert, increasing security at transport hubs, religious sites, media offices and department stores as the search for the assailants got under way.

In a televised address to the nation, French president Francois Hollande said everything would be done to bring the attackers to justice. He declared a national day of morning would take place on Thursday and urged French people to continue to be themselves, to be united in their response to the attack.

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Earlier, Mr Hollande rushed to the scene of what appeared to be a carefully-planned attack. “An act of indescribable barbarity has just been committed today in Paris,” he said. “Measures have been taken to find those responsible, they will be hunted for as long as it takes to catch them and bring them to justice.”

World leaders including US president Barack Obama and German chancellor Angela Merkel condemned the attack.

Supporters of the militant Islamic State group celebrated the killings, describing them as revenge against France.

Shortly before 11.00am the AFP news agency reported shots had been fired at the offices of Charlie Hebdo on Boulevard Richard Lenoir.

Witnesses said a number of masked men armed with automatic weapons went to the second floor and started firing indiscriminately in the newsroom.

There were unconfirmed reports that one of the men shouted “Allahu Akbar” before fleeing in a black getaway car which waited at the end of the street.

An amateur video shows one of two hooded man shooting dead a wounded policeman at point-blank range with a rifle. In another clip the men are heard shouting in French: “We have killed Charlie Hebdo. We have avenged the Prophet Mohammad.”

Five of the victims have been named, including four Charlie Hebdo journalists: editor Stéphane Charbonnier and cartoonists Jean Cabut, Georges Wolinski and Bernard Verlhac. AFP reported that Bernard Maris, an economist and writer who contributed to the magazine, was also killed. The attack took place during the magazine’s daily editorial meeting.

The attack drew swift international condemnation. German Chancellor Angela Merkel said it was an attack on freedom of speech and the press. British prime minister said it was “sickening”.

The White House said France had been one of the stalwart allies of the United States in the fight against Islamic State.

Thousands gathered in cities across Europe as night fell on Wednesday to take part in vigils for the victims of the Paris attack.

The shootings came amid mounting tension about immigration in France and what many non-Muslim French see as rising Islamic influence in society. The Islamic State group has repeatedly threatened to attack France.

Cherlie Hebdo is well known for courting controversy with satirical attacks on political and religious leaders and has published numerous cartoons ridiculing the Prophet Mohammad.

Its cover story this week featured Michel Houellebecq, the controversial author whose latest book Soumission, or Submission, portrays France in 2022 run by a Muslim president, according to the laws of conservative Islam.

Additional reporting: agencies