French President Hollande meets Obama to discuss Islamic State

French President Francois Hollande has arrived in Washington to seek support for his newly declared war on the Islamic State group, while the US has announced a worldwide travel alert for its citizens.

As Hollande on Tuesday prepared for talks with President Barack Obama on the campaign against IS in Syria and Iraq, the manhunt continued in Europe for Belgian-born Salah Abdeslam, a key suspect in the Paris attacks that killed 130 people.

As Brussels entered a fourth day of lockdown over fears of an "imminent" attack, relations between Russia and Turkey - two countries seen as key to resolving the Syria conflict - were plunged into crisis by Turkey's downing of a Russian warplane.

Meanwhile, 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 in the Montrouge suburb of the capital.

Telephone data placed Abdeslam in the area on November 13, the night of the attacks.

Brussels will stay on its highest level of alert at least until next Monday, with Belgian Prime Minister Charles Michel warning that the threat "remains serious and imminent", though schools and the Metro train system will re-open Wednesday.

The security measures include city-wide patrols of armed soldiers and police, not seen in Brussels for two decades.

Belgian authorities have charged a fourth person in connection with the bloodshed in Paris, which was claimed by the Islamic State group.

Hollande's trip to Washington is part of a frantic week of shuttle diplomacy by the French leader as he tries to rally global support for increased strikes against IS.

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

The UN Security Council last week authorised "all necessary measures" to fight IS.

In Brussels, an eerie atmosphere hung over the city with soldiers in camouflage patrolling everywhere from railway stations to EU institutions.

France has said it will install security gates at stations in Paris and Lille for Thalys cross-Europe rail services by December 20.

A Thalys train from Amsterdam to Paris was attacked by a heavily armed man in August, but he was overpowered by passengers.

In Brussels' normally bustling historic Grand Place, a few bars and restaurants were open for business but they were struggling to find customers.

The only real activity downtown was deliverymen offloading crates for near-empty shops as builders hammered together stalls for a Christmas market meant to open on Friday.