PARIS — A fast-moving fire that appeared to be accidental swept through a birthday party in a basement bar in northwest France, killing at least 13 people and injuring six others, authorities said Saturday.

More than 80 firefighters battled the early Saturday blaze at the downtown Cuba Libre bar in the city of Rouen, Mayor Yvon Robert said, calling the fire “very brief.”

The birthday party was “a moment of joy for those involved that … ended tragically,” Robert said.

Vice prosecutor Laurent Labadie said that the first testimonies from survivors and the early police investigation indicated the “fire was completely accidental.”

“There was no explosion,” Labadie said. “Candles on a birthday cake started the fire after the person who carried it tripped on the stairs leading to the basement.”

Labadie said it’s still unclear how many people were partying at the Cuba Libre club, adding that most of the dead were between 18 and 25 years old.

In a statement released by the French presidency, Francois Hollande expressed his “solidarity” with the victims’ families and vowed that the ongoing judicial investigation will shed light on the “the causes of this dramatic accident.”

Labadie said sound-insulating material on the basement’s walls quickly ignited and party guests had no time to escape from the basement.

At the scene of the fire, residents paid tribute to the victims by laying flowers. Images on French television from outside the bar showed a large ground-floor window broken open, burned red bar chairs and a tattered awning.