The French agency in charge of emergency management on Monday shot down President Trump’s suggestion that “flying water tankers” could help put out the blaze at Paris’s Notre Dame Cathedral, noting that water bombing could cause the entire building to collapse.

Trump had proposed the idea in an afternoon tweet shortly after TV networks began airing images of the historic cathedral engulfed in flames.

“So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!” Trump tweeted.

He also mentioned the cathedral in remarks at an event in Minnesota, telling the crowd, “They don’t know what caused it. They say renovation, and I hope that’s the reason. Renovation? What’s that all about?”

Not long after the president concluded his speech, the French civil defense department sent out tweets — three in French, followed by one in English — that made no mention of Trump but described the infeasibility of using aircraft to douse the fire.

“Hundreds of firemen of the Paris Fire Brigade are doing everything they can to bring the terrible #NotreDame fire under control,” the department said in its English-language tweet. “All means are being used, except for water-bombing aircrafts which, if used, could lead to the collapse of the entire structure of the cathedral.”

The cathedral has stood at the heart of Paris for more than 800 years.

Cathedral spokesman André Finot told French media that the building had suffered “colossal damage.” But later Monday, officials said that the iconic twin towers that frame the cathedral’s grand entry had been saved and that the building’s exterior structure had been preserved.

There are unique challenges to extinguishing a fire in a cathedral, according to U.S. Fire Administrator Keith Bryant.

A fire must have fuel, air and heat to survive. To extinguish one, he explained, firefighters must “disrupt the process of combustion.”

The cathedral is a massive structure with large and open space, providing a fire with enough oxygen to support itself. It’s also filled with combustible material — like wood, paper and cloth — and it requires tremendous amounts of water to either control or extinguish such a fire.

“Any structure like that is going to create a huge challenge,” he said, noting that he did not know the specific capabilities of the Parisian fire force. “The fire spread itself could be very rapid and overwhelm the firefighting efforts,” he said.

Water is the best extinguishing agent because it cools the fire and takes away its heat source, but hoses are limited in how much water they can pump; at full capacity, U.S. fire engines can release 1,200 to 1,500 gallons per minute.

James McAuley in Paris contributed to this report.