British Prime Minister David Cameron on Wednesday condemned the attack on the office of a Paris magazine, pledging solidarity with the French people against "all forms of terrorism."

At least 12 people were killed in a shooting at the headquarters of French newspaper Charlie Hebdo Wednesday morning, French media reported.

"The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press," Cameron said on his Twitter account.

"I am sure the whole House will join me in condemning the barbaric attack this morning on an office of a magazine in Paris," he told the British Parliament in his weekly Prime Minister's Questions (PMQs).

Reaffirming his solidarity with the French people, he added: "We stand united with the French people in our opposition to all forms of terrorism and stand squarely for free speech and democracy."

Two or three men, heavily armed, attacked the headquarters of the newspaper Charlie Hebdo in Paris's 11th arrondissement, said France Info.

French president Francois Hollande is to hold an emergency cabinet meeting on Wednesday afternoon in Elysee.

Charlie Hebdo was firebombed in 2011, due to publishing a controversial series of cartoons depicting the Prophet Mohammed.