President Donald Trump, in Minnesota for a Tax Day event, called the incident at Notre Dame in Paris a "terrible, terrible fire" that was likely caused by renovations.
"When we left the plane it was burning at a level that you rarely see a fire burn," he said.
The fire rapidly spread and took over the iconic cathedral as onlookers scrambled to get a view of the destruction, even as soot and cinder fell onto them. Firefighters atop cherry picker cranes sprayed water onto the church in an attempt to calm the flames.
It was not immediately known what caused the fire and whether anybody was injured. The cathedral has been surrounded by scaffolding and was under construction.
The President lamented the damage being done to the historic cathedral.
"That's beyond countries, that's beyond anything," he said. "It's a part of our culture."
He said the fire would put "a damper" on the tax round table he was about to begin.
Earlier Monday, he weighed in on Twitter with an unsolicited suggestion for Parisian firefighters.
"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!" he tweeted.
Vice President Mike Pence and first lady Melania Trump also tweeted on the fire.
"Notre Dame is an iconic symbol of faith to people all over the world -- and it is heartbreaking to see a house of God in flames. Our thoughts and prayers are with the firefighters on the scene and all the people of Paris," he said.
And the first lady said she was "praying for everyone's safety."
"My heart breaks for the people of Paris after seeing the fire at Notre Dame Cathedral," she said.