UPDATED 11.08am: France and the United States have pledged to step up the fight against the Islamic State group, urging Russia to throw its weight behind global efforts to resolve the four-year Syria conflict.

President Francois Hollande met his US counterpart Barack Obama at the White House on Tuesday as Turkey's downing of a Russian warplane dealt a severe blow to efforts to coordinate the fight against IS.

Speaking 11 days after jihadists killed 130 people in the French capital, Hollande urged an "implacable" joint response to crush the group in Syria and Iraq.

At a joint press conference, Obama pledged America's full support in the wake of the November 13 carnage, switching into Hollande's language to tell him: "We are all French."

"We cannot & we will not succumb to fear. Nor can we allow fear to divide us. For that’s how terrorists win” —@POTUS https://t.co/Y2Rrhi1f22

— The White House (@WhiteHouse) November 24, 2015

"We are here to declare that the United States and France stand united in total solidarity to deliver justice to terrorists and those who sent them, and to defend our nations," Obama said.

"Americans will not be terrorised."

Washington and Paris have both stepped up their fight against IS in Syria, with France launching its first strikes from the Charles de Gaulle aircraft carrier in the Mediterranean on Monday and the US calling for more international cooperation against the jihadist group.

Hollande said he and Obama had agreed to "scale up our strikes both in Syria and in Iraq to broaden our scope to strengthen our intelligence sharing regarding the targets."
Both said they would boost support for forces battling IS on the ground - while both continuing to rule out any ground intervention.

"France will not intervene militarily on the ground," Hollande said. "It is for the local forces to do so."

The leaders' talks in Washington came as Turkey's downing of a Russian warplane at the Syrian border threatened to dramatically fan tensions in the volatile region.

The most serious incident involving Russian forces since they entered the conflict in support of President Bashar Al-Assad - the downing drew a furious response from President Vladimir Putin who accused NATO-member Turkey of "a stab in the back."

Obama and Hollande joined UN Secretary-General Ban Ki-moon in warning against any escalation.

"I think it is very important for us to right now make sure that both the Russians and the Turks are talking to each other and find out exactly what happened, and take measures to discourage any kind of escalation," Obama told reporters.

Hollande was in Washington as part of a frantic week of shuttle diplomacy as he tries to rally global support for increased strikes against IS, which claimed the Paris attacks.
Both Hollande and Obama reiterated their determination to see Assad step down in order to give Syria a chance for peace, with Hollande saying "it should be as soon as possible."

The French leader will hold talks with German Chancellor Angela Merkel in Paris on Wednesday and with Putin in Moscow on Thursday, before dining with Chinese President Xi Jinping in the French capital on Sunday.

Police in France said they were analysing what is thought to be a suicide belt similar to those used in the Paris attacks, found without its detonator in a dustbin outside the capital.

Telephone data placed key suspect Salah Abdeslam in the area the night of the attacks.
Across the border in Belgium, Brussels entered a fourth day of lockdown over fears of an "imminent" terror strike as the manhunt continued for the Belgian-born Abdeslam.

As the search intensified for the 26-year-old suspect, authorities said a fourth person has been charged in connection with the bloodshed in Paris.

Two others - Mohammed Amri, 27, and Hamza Attou, 20 - were charged on Monday on suspicion of helping Abdeslam escape to Brussels after the attacks, while a third unnamed person faces charges of aiding him.

France has launched a major security crackdown since the attacks with police searching more than 1200 premises, arresting 165 people and seizing 230 weapons - including what the interior minister called "weapons of war."