An investigation into the November 13 attacks in Paris widened on Tuesday when French prosecutors said a man who provided lodging to the suspected ringleader must have known of a militant plot, and Belgium issued a warrant for a new suspect.

Painting a chilling picture of ringleader Abdelhamid Abaaoud, the Paris prosecutor said that after dropping off the gunmen and suicide bombers at the cafes and bars where the attacks were to take place, he later returned to the scene while the killing spree was in full swing.

The coordinated attacks, in which 130 people were killed, prompted France to declare a national state of emergency and to step up air strikes in Syria on Islamic State, the militant group that has claimed responsibility.

President Francois Hollande, seeking to rally global support for the military campaign against Islamic State, met with US President Barack Obama at the White House on Tuesday where they agreed to scale up operations against Islamic State in Syria and Iraq.

At a joint news conference, Hollande said he and Obama shared a "relentless determination to fight terrorism anywhere and everywhere."

In Paris, prosecutor Francois Molins said Islamist militants who died during a shootout with police on Nov. 18 had been plotting an attack on the capital's business district. Reuters exclusively reported the plot to attack the district of La Defense on Nov. 18.

Molins said he had put under formal investigation a Frenchman who had provided lodging for Abaaoud and his associates at the apartment in the suburb of St. Denis.

"Jawad Bendaoud himself welcomed the terrorists on Nov. 17 towards 10:45 p.m. He could not have been in any doubt ... that he was taking part in a terrorist organisation," Molins told a news conference.

Bendaoud said that before he was detained by police last Wednesday he had been asked to put up two people for three days in the apartment, but had no idea one of them may have been the suspected mastermind of the Nov. 13 attacks.

Abaaoud died during the police raid along with Hasna Aitboulahcen, a woman believed to be his cousin, and an as yet unidentified third person.

BRUSSELS LOCKDOWN

French investigators are still piecing together exactly who did what when and have launched a hunt to find Salah Abdeslam, who is suspected of being the eighth attacker mentioned by Islamic State when it claimed responsibility.

Abdeslam, 26, fled to Belgium the day after the shootings and his presumed presence in Brussels was one of the factors behind a security lockdown in the city over the past few days.

Fearing an imminent Paris-style attack, Belgium has extended a maximum security alert in Brussels until next Monday. About half the stations on its metro system will reopen on Wednesday along with city schools, but 300 additional police officers and 200 soldiers will be deployed.

Belgium has been at the heart of investigations into the attacks since France said two of the suicide bombers in Paris had lived there. Five people, including two who travelled with Abdeslam back to Brussels, have been charged with terrorist offences in Belgium. Abdeslam's brother, Brahim, blew himself up.

Belgium's state prosecutor, in a statement announcing details of other people charged in the case, said on Tuesday it had issued an international arrest warrant for Mohamed Abrini, who was seen with Abdeslam two days before the attacks.

Abrini, 30, was filmed with Abdeslam at a fuel station in northern France on Nov. 11 and was driving the Renault Clio car later used by the attackers in the French capital.

A police wanted poster described Abrini as "dangerous and probably armed".

While major shopping centres in Brussels remained closed on Tuesday, two Ikea furniture stores on the edge of town reopened, along with some of the larger supermarkets in the city.

The Magritte museum remained shut, however, and Brussels had yet to decide whether to open its Christmas market on Friday in the historic Grand Place, where workers have set up stalls with an armoured personnel carrier in the background.

"We are at the time of year when we are supposed to have a lot of people, and increase business," said Brussels toy shop worker Laeticia Shalaj. "People are scared and are afraid of leaving their homes."

Since the Paris killings, France has moved its flagship Charles de Gaulle aircraft carrier into the eastern Mediterranean to step up its bombardments of Islamic State targets in Syria and Iraq.

France said late on Tuesday its warplanes had destroyed an Islamic State command centre at Tal Afar, some 45 km (28 miles) west of Mosul in Iraq.

After talks with Hollande at the White House, Obama said: "We are here today to declare that the United States and France stand united in total solidarity to deliver justice to these terrorists and those who sent them and to defend our nations."

As millions of Americans prepare to travel for the US Thanksgiving holiday on Thursday, Obama acknowledged fear among Americans of a Paris-style attack in the United States.

The White House said Obama was set to meet on Wednesday morning with his national security advisers to review the "homeland security posture" of the United States after the Paris attacks and "as we enter the holiday season.

"The president was briefed that there is currently no specific, credible threat to the homeland" from Islamic State, the White House said.

Hollande is due to visit Moscow on Thursday, where he and Russian President Vladimir Putin are expected to discuss ways of boosting their campaign to crush Islamic State.

EXPLOSIVE BELT?

Tracing Abdeslam's movements since the attacks has been a main focus of the investigations in Paris. An explosive belt was found dumped near the capital on Monday, close to a location where his mobile phone was detected the night after the attacks, the prosecutor said.

Molins said the belt was similar to those used by the bombers during the Nov. 13 attacks, although it was still being established on Tuesday whether the belt was Abdeslam's.

One theory was that Abdeslam had intended to blow himself up in the 18th district of Paris but had abandoned the plan, although it was not clear why. – Reuters