French police have killed three gunmen, including two brothers suspected of shooting dead 12 people in an attack at the Paris offices of the satirical newspaper Charlie Hebdo.

Cherif and Said Kouachi were at the centre of a stand-off at a printing business in an industrial area in Dammartin-en-Goele, around 40 kilometres from an area which was subject to intense police searches on Thursday.

Police stormed the building several hours after a car chase that saw shots fired. A hostage was freed unhurt. It has been reported the gunmen came out firing when police moved in.

In a simultaneous assault, dozens of heavily armed officers stormed a kosher grocery store in eastern Paris where a gunman with links to the same Islamist group as the Kouachi brothers had taken several people hostage.

Explosions were heard shortly before television pictures showed a number of people being freed.

The gunman was shot dead and four hostages were killed. Two officers were injured as police forced their way into the store.

Smoke, in the left of picture, is seen as French special forces launch their assault at a kosher supermarket where several people were taken hostage. ( Reuters: Gonzalo Fuentes )

After police brought the sieges to an end, French president Francois Hollande called for unity and vigilance from his countrymen.

"With the prime minister I have further strengthened the means to protect public places. But we must be vigilant. I also call upon unity because as I have expressed before... it is our best weapon," he said.

"Today in a kosher supermarket it was a terrible act that was committed. The people who committed the acts, these fanatics, have got nothing to do with the Muslim religion."

Mr Hollande has invited a number of world leaders to a "great gathering" on Sunday, when he has called for "all French men and women to rise".

Police said the gunman in the grocery store was believed to be connected to the killing of a policewoman at a metro station on Thursday and released mugshots of Amedy Coulibaly, 32, and Hayat Boumeddiene, 26.

Coulibaly knew at least one of the suspects in the Charlie Hebdo massacre, a source told French news agency Agency France-Press.

It is understood police are still searching for Boumeddiene.

Hayat Boumeddiene (left) and Amedy Coulibaly are suspects in the murder of a policewoman. ( Prefecture de Police )

The exact number of hostages in the grocery store siege was unclear, but local media reported at least five were held.

A security source said Coulibaly shot two people as soon as he entered the store, and had "one or more bags" of explosives with him but did not detonate them.

He had tried to booby-trap the entrance to the shop "but had not hooked up the explosives", the source added.

Police cordoned off the area in Port de Vincennes and interior minister Bernard Cazeneuve rushed to the scene.

Reports have surfaced that Coulibaly and Cherif Kouachi were known to each other and were seen together in 2010 while visiting another jihadist, Djamel Beghal, the mastermind of a failed prison break-out plot.

Coulibaly was convicted for his role in the planned break-out and was well-known to anti-terrorist police.

Plan to 'neutralise' Charlie Hebdo gunmen

On Wednesday the two Kouchai brothers had stormed the offices of satirical magazine Charlie Hebdo armed with Kalashnikov rifles.

They shot dead eight journalists, two police officers, a maintenance man and a visitor to the offices in the worst militant attack on French soil in decades.

The French government then launched a massive manhunt for the suspects, deploying 88,000 soldiers and police.

Mr Cazeneuve confirmed police had cornered the Kouachi brothers in Dammartin-en-Goele.

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"An operation is underway which is set to neutralise the perpetrators of the cowardly attack carried out two days ago," he said in a televised statement.

Helicopters were seen flying over an industrial zone outside the town where around 200 children from a pre-school, primary school and high school were evacuated to the town gymnasium.

Earlier, police had chased a vehicle at high speed along the A2 motorway towards Paris as authorities appeared to be closing in on the two brothers. Parts of the chase were broadcast live on TV as media vehicles followed closely behind police.

Gunshots rang out and police trucks, ambulances and armoured vehicles descended on the area close to Charles de Gaulle airport.

The Paris prosecutor denied French media reports that at least one person had been killed when the shots were fired during the car chase.

The previous night officers had focused their search on the woodland village of Corcy, not far from a service station where police sources said the brothers had been seen in ski masks.

Said and Cherif Kouachi were French-born sons of Algerian-born parents, both in their early 30s, and both were already under police surveillance.

The gunmen were cornered by police inside the printing business in an industrial area north-east of Paris. ( Google )

One was jailed for 18 months for trying to travel to Iraq a decade ago to fight as part of an Islamist cell.

Police had described them as being "armed and dangerous".

US and European sources close to the investigation said on Thursday that 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 were listed in two US 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 Centre, 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 Centre, said he could neither confirm nor deny if the Kouachis were listed in counter-terrorism databases.

Reuters/AFP