At least one hostage was seized in a town northeast of Paris on Friday during a huge manhunt for two brothers suspected of killing 12 people at a satirical weekly, according to a police source.

Five helicopters were seen flying over an industrial zone outside the town of Dammartin-en-Goele and the French Interior Minister confirmed an operation was taking place there. A police source said the two suspects had been sighted in the town, where at least one person was taken hostage.

Before night fell on Thursday, officers had been focusing on their search some 40 km away on the woodland village of Corcy, not far from a service station where police sources said the brothers had been sighted in ski masks a day after the shootings at the newspaper.

The fugitive suspects are French-born sons of Algerian-born parents, both in their early 30s, and already under police surveillance. One was jailed for 18 months for trying to travel to Iraq a decade ago to fight as part of an Islamist cell. Police said they were "armed and dangerous".

US and European sources close to the investigation said on Thursday that one of the brothers,Said Kouachi, was in Yemen in 2011 for several months training with Al Qaeda in the Arabian Peninsula (AQAP), one of the group's most active affiliates.

A Yemeni official familiar with the matter said the Yemen government was aware of the possibility of a connection between Said Kouachi and AQAP, and was looking into any possible links.

US government sources said Said Kouachi and his brother Cherif Kouachi were listed in two U.S. security databases, a highly classified database containing information on 1.2 million possible counter-terrorism suspects, called TIDE, and the much smaller "no fly" list maintained by the Terrorist Screening Center, an interagency unit.

US television network ABC reported that the brothers had been listed in the databases for "years."

Dave Joly, a spokesman for the Terrorist Screening Center, said he could neither confirm nor deny if the Kouachis were listed in counter-terrorism databases.

While world leaders described Wednesday's attack on the weekly newspaper Cahrlie Hebdo as an assault on democracy, Al Qaeda's North Africa branch praised the gunmen as "knight(s) of truth".

Charlie Hebdo, where journalists were gunned down during an editorial meeting, had been firebombed in the past for printing cartoons that poked fun at militant Islam and some that mocked the Prophet Muhammad.

Two of those killed were police posted to protect the paper.

On Thursday, U.S. President Barack Obama made an unannounced visit to the French Embassy in Washington to pay his respects.

He wrote in a condolence book: "As allies across the centuries, we stand united with our French brothers to ensure that justice is done and our way of life is defended. We go forward together knowing that terror is no match for freedom and ideals we stand for - ideals that light the world."

Amid local media reports of isolated incidents of violence directed at Muslims in France, President Francois Hollande and his Socialist government have called on the French not to blame the Islam faith for the Charlie Hebdo killings.

Hollande has held talks with opposition leaders and, in a rare move, was due to invite Marine Le Pen, leader of the resurgent anti-immigrant National Front, to his Elysee Palace for discussions on Friday.

MOURNING

Bewildered and tearful French people held a national day of mourning on Thursday. The bells of Notre Dame pealed for those killed in the attack on the left-leaning slayer of sacred cows whose cartoonists have been national figures since the Parisian counter-cultural heyday of the 1960s and 1970s.

Many European newspapers either re-published Charlie Hebdo cartoons or lampooned the killers with images of their own.

Searches were taking place in Corcy and the nearby village of Longpont, set in thick forest and boggy marshland about 70 km north of Paris, but it was not clear whether the fugitives who had been spotted in the area were holed up or had moved on.

Corcy residents looked bewildered as heavily armed policeman in ski masks and helmets combed the village meticulously from houses to garages and barns.

"We're hearing that the men could be in the forest, but there's no information so we're watching television to see," said Corcy villager Jacques.

In neighbouring Longpont, a resident said police had told villagers to stay indoors because the gunmen may have abandoned their car there. Anti-terrorism officers pulled back as darkness fell. The silence was broken by the sound of a forest owl.

Prime Minister Manuel Valls, asked on RTL radio on Thursday whether he feared a further attack, said: "That's obviously our main concern and that is why thousands of police and investigators have been mobilised to catch these individuals."

SUSPECT JAILED

Police released photographs of the two suspects, Cherif and Said Kouachi, 32 and 34. The brothers were born in eastern Paris and grew up in an orphanage in the western city of Rennes after their parents died.

The younger brother's jail sentence for trying to fight in Iraq a decade ago, and more recent tangles with the authorities over suspected involvement in militant plots, raised questions over whether police could have done more to watch them.

Cherif Kouachi was arrested on January 25, 2005 preparing to fly to Syria en route to Iraq. He served 18 months of a three-year sentence.

"He was part of a group of young people who were a little lost, confused, not really fanatics in the proper sense of the word," lawyer Vincent Ollivier, who represented Cherif in the case, told Liberation daily.

In 2010 he was suspected of being part of a group that tried to break from prison Smain Ali Belkacem, a militant jailed for the 1995 bombings of Paris train and metro stations that killed eight people and wounded 120. The case against Cherif Kouachi was dismissed for lack of evidence.

In the wake of the killings, authorities tightened security at transport hubs, religious sites, media offices and stores. Police also increased their presence at entry points to Paris.

The defence ministry said it sent 200 extra soldiers from parachute regiments across the country to help guard Paris.

Le Figaro newspaper reported that the interior ministry had been inundated with dozens of requests for police protection from "personalities feeling in danger", citing a high-ranking police official. - Reuters