Washington (AFP) - France and the United States pledged on Tuesday to step up the fight against the Islamic State group, urging Russia to throw its weight behind global efforts to resolve the four-year Syrian conflict.

President Francois Hollande met his US counterpart Barack Obama at the White House 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 solidarity in the wake of the November 13 carnage, switching into Hollande's language to tell him, "We are all French."

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 and the US calling for more international cooperation against the jihadists.

While announcing no specific new action, Hollande said he and Obama 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 leaders said they would boost support for forces battling IS on the ground -- while continuing to rule out any ground campaign.

"France will not intervene militarily on the ground," Hollande said.

- Influence on Assad -

Illustrating the leaders' message on tighter cooperation, France said its warplanes had hit an IS command center near its key western Iraqi stronghold of Mosul, in a strike led with the US Air Force.

The 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."

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

In a call, Obama and Turkish leader Recep Tayyip Erdogan "agreed on the importance of de-escalating the situation and pursuing arrangements to ensure that such incidents do not happen again," the White House said.

- Shuttle diplomacy -

The US military has backed up Turkey's claim that Turkish pilots warned the Russian jet 10 times -- but failed to get a response -- before shooting it down.

Obama said NATO ally Turkey had a right to defend its airspace -- but also appealed to Russia to engage at the side of the 65 countries battling IS in Syria.

"Given Russia's military capabilities and the influence they have on the Assad regime, them cooperating would be enormously helpful in bringing about resolution of the civil war in Syria," Obama said.

"If and when they do, it will make it easier for us to go after ISIL."

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."

"He has been the problem -- he cannot be the solution," Hollande said.

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.

- Manhunt -

Acting on a French resolution, the UN Security Council last week authorized "all necessary measures" to fight IS.

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

But the delicate diplomacy around the conflict was thrown brutally off course by the fighter jet downing, as Putin warned of "serious consequences for Russian-Turkish relations."

The US government has issued a worldwide travel alert warning American citizens of "increased terrorist threats" in the wake of Islamist attacks in Paris, Mali, Turkey and elsewhere.

But during a meeting with his national security team, Obama was briefed that there is "no specific, credible threat to the homeland from ISIL," the White House said.

Paris prosecutor Francois Molins meanwhile said the suspected ringleader of the deadly assaults in the French capital and an accomplice planned to carry out a suicide attack on the city's La Defense business district the following week.

French police said they were also analyzing 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.

The suspected ringleader of the deadly Paris attacks and an accomplice planned to carry out a suicide attack on the city's La Defense business district the following week, the chief prosecutor said Tuesday.

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.

A fifth person was charged in Belgium in connection with the bloodshed in Paris.

Since the attacks, French police have searched more than 1,200 premises, arresting 165 people and seizing 230 weapons -- including what the interior minister called "weapons of war."