Paris (CNN)The next 90 minutes will be crucial to efforts to save what remains of the iconic Notre Dame cathedral amid a devastating fire on Monday, a Paris fire official said. The fire, which has been burning for several hours, has already caused the collapse of the spire towering above the cathedral. Jean-Claude Gallet, the Paris Fire Brigade commander general, said about 10 p.m. that the next risk is if the great bells falls because of the flames. "If these bells fall, it's the tower that collapses. There are firefighters inside and outside. The next hour and a half will be crucial," he said to reporters on the scene, regarding the fire threatening the north tower. "We need to win this battle and block the spreading of the flames," he said. "The most efficient action is from the inside. We are not sure if we will be able to stop the spreading of the flames to the North tower." Too soon?