Britain has stepped up security at ports serving France following the Islamist massacre in Paris, officials said Thursday, as the MI5 intelligence agency offered French counterparts its "full support."

The terror threat level remains at "severe" and officials said the measures were a precautionary measure following the slaying of 12 people at the French satirical magazine Charlie Hebdo on Wednesday.

Elite French security forces tightened the net Thursday on two brothers suspected of slaughtering 12 people in an Islamist attack after discovering an abandoned getaway car in a northeastern town.

Helicopters buzzed overhead as police mounted a frantic manhunt for the two fugitives thought to be behind the bloodbath at Charlie Hebdo magazine in Paris, the worst terrorist attack in France for half a century.

Muslim places of worship in two French towns were fired upon overnight, leaving no casualties, prosecutors said on Thursday.

Three blank grenades were thrown at a mosque shortly after midnight in the city of Le Mans, west of Paris. A bullet hole was also found in a window of the mosque.

A policewoman who was shot by a gunman wearing a bullet-proof vest just outside Paris has died and a second victim is in serious condition, police said Thursday.

The man escaped after the attack on Thursday morning, which comes just a day after a deadly Islamist assault on satirical magazine Charlie Hebdo left 12 dead, although no link has yet been established between the two incidents.

France must review its policy towards the Middle East and the Muslim world following the deadly attack on the Charlie Hebdo satirical magazine in Paris, Iran's press said Thursday.

Several newspapers in the Islamic republic linked the attack to France's support for Syria's armed opposition and its participation in the international coalition waging air strikes against the Islamic State jihadist group.

Cherif Kouachi, the 32-year-old hunted along with his older brother Said for the attack on satirical weekly Charlie Hebdo is a jihadist who has been well-known to anti-terror police for many years. 

Cherif, who was born on November 28, 1982 in Paris not far from where the attack took place, had already been jailed in 2008 for his role in sending fighters to Iraq.

French security forces were on Thursday desperately hunting two brothers suspected of gunning down 12 people in an Islamist attack on a satirical weekly, as a stunned and outraged France mourned the victims.

Wednesday's massacre at the Charlie Hebdo magazine triggered poignant and spontaneous demonstrations of solidarity around the world and more than 100,000 poured onto the streets of France.

Hundreds in Washington, New York and Canada rallied Wednesday night in the frigid cold proclaiming "I am Charlie" in solidarity with those killed in a deadly attack on French magazine Charlie Hebdo.

Some 200 to 300 French and American protesters gathered in the nation's capital in front of the Newseum, a museum dedicated to news, waving signs and French flags emblazoned with the now ubiquitous rallying cry "Je suis Charlie."

More than 100,000 people gathered across France to pay tribute to the victims of Wednesday's massacre by Islamist gunmen in Paris, as thousands also rallied in other European cities and the "I Am Charlie" hashtag swept the Internet.

With the gunmen still on the loose in Paris after killing 12 people, defiant crowds swarmed into the Place de la Republique, barely a kilometer (half a mile) from the scene of the bloodbath at the Charlie Hebdo satirical weekly.

Israeli Prime Minister Benjamin Netanyahu called for global unity to fight the "scourge" of radical Islam, after an  attack on a satirical French newspaper killed 12 people on Wednesday.

Speaking at a crisis center in Jerusalem set up to deal with severe winter storms battering the Middle East, he expressed Israel's condolences for the "brutal act in the heart of Paris today."