Rouen (France) (AFP) - A fire sparked by birthday cake candles tore through a bar in northern France early Saturday, killing at least 13 people in the nation's deadliest blaze in over a decade, investigators said.

Through the shattered front windows of the Au Cuba Libre bar in Rouen melted stools and scorched liquor bottles were visible, as tearful mourners hugged each other and brought flowers to the scene.

The dead were aged between 16 and 25. One of the six injured was in a life threatening condition in hospital.

Authorities said the bar had been crowded with young partygoers.

Prime Minister Manuel Valls expressed "deep sadness at the tragedy that claimed 13 young lives".

Local deputy prosecutor Laurent Labadie told AFP the blaze, which began around midnight (2200 GMT Friday) in the bar's basement, was triggered accidentally.

"Someone came down with a birthday cake with candles and fell down the stairs," Labadie said, citing initial investigation findings.

"The candles were thrown onto the walls and ceiling, where there was soundproofing material. There was an immediate conflagration and gas was released."

Firefighters were soon on the scene but those inside the bar were battling deadly fumes as well as the flames.

Police said that the fire had ignited polystyrene on the ceiling of the basement room, releasing a toxic mix of gases that poisoned the victims.

- 'Like a flame-thrower' -

One witness, who gave her name as Stephanie, 36, described the moment the fire took hold.

"I was at the bar on the ground floor having a drink, when we saw the flames -- it was like a flame-thrower, everything happened so fast," she told reporters.

One of the injured was in a critical condition, the secretary general of the Seine-Maritime prefecture Yvan Cordier told AFP.

French President Francois Hollande voiced his sympathy for the victims' families and said "everything would be done" to find out what caused the fire.

The investigation will focus on whether building safety rules were respected at the bar, particularly in relation to fire exits and flammable materials.

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Under French building regulations, polystyrene panels are banned from use on ceilings, and bars like the Au Cuba Libre must be equipped with at least one portable fire extinguisher.

But a source at the crisis cell of the French Interior Ministry said that "unfortunately there are always people who try to get round the rules between safety inspections, which are sometimes pre-planned, sometimes unannounced".

The bar, in a busy part of town and no more than 200 metres (yards) from the River Seine was very popular with young people.

"It was more than a bar, it was family," said one 20-year-old who gave his name as Willy, standing disconsolately outside the building's ruined facade.

"It's where we went to be all together," he added.

The mother of an 18-year-old girl who died in the blaze said she had asked her daughter not to go to the party.

"But she didn't listen" the weeping woman explained, adding that her daughter had wanted to become a nurse.

- 'Afraid was another attack' -

The blaze in Rouen was France's deadliest since September 2005 when an apartment building fire in the Paris suburb of L'Hay-les-Roses killed 18 people.

It is the highest death toll in a French night spot fire since November 1970, when a blaze at a disco in the east of the country claimed the lives of 146 people, most in their early 20s.

Jittery France is on edge after a series of recent attacks by Islamic State-affiliated jihadists, including the killing of an 85-year-old priest in nearby Saint-Etienne-du-Rouvray.

"I was afraid that it was another attack, but right away I was told that it was an accident," witness Rachid Ahmaymi, 36, told AFP. "Still I didn't sleep last night and have come back to hear the latest news."

The priest's funeral was held in Rouen's cathedral and was attended by more than 2,000 mourners shocked by the brutal killing of the elderly clergyman.

The church attack came less than two weeks after another assailant ploughed a 19-tonne truck into a crowd celebrating Bastille Day in the Riviera city of Nice, killing 85 people and wounding more than 300 others.