As a catastrophic inferno tore through a cultural treasure, US President Donald Trump assessed the response from 6440km away and offered unsolicited advice for firefighters trying to save Paris' Notre Dame Cathedral.

"Perhaps flying water tankers could be used to put it out," Trump tweeted from Air Force One, minutes before landing in Minnesota for a Tax Day speech. "Must act quickly!"

Trump's armchair observations were striking, even for a president who seems to offer an opinion about just about everything.

Just hours before, Trump had weighed in on Boeing's ongoing troubles after two crashes of its new airliner, suggesting a rebranding campaign is in order for the beleaguered 737 Max, and he'd spent the previous day commentating on professional golf as Tiger Woods overcame long odds to win the Masters.

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"They're having a terrible, terrible fire," Trump said during his remarks in Minnesota, describing watching the flames on television from the presidential aircraft.

"It looks like it's burning to the ground."

Firefighters in Paris ruled out dropping water from the air.

In what appeared to be a direct jibe at Trump's firefighting suggestion, France's civil security agency posted its only English-language message during the incident to defend its handling of the fire: "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."

About 400 firefighters were working around the building, according to the Ministry of the Interior.

"We are more efficient inside," General Jean-Claude Gallet, commander of the Paris fire brigade said.

The fire, which spread extremely fast, took hold in the attic of the cathedral, firefighters said.

The disaster appeared to resonate with the US President, who called Notre Dame "one of the great treasures of the world" and declared it to be "beyond countries, that's beyond anything. It's a part of our growing up, it's a part of our culture, it's a part of our lives."

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On his way back to Washington, he tweeted, "God bless the people of France!"

Firefighters have been seen tens of metres up in the air using hoses to fight the huge blaze in Notre-Dame Cathedral. Photo / AP
Firefighters have been seen tens of metres up in the air using hoses to fight the huge blaze in Notre-Dame Cathedral. Photo / AP
The fire in Notre-Dame Cathedral began about 4.50am NZT on Tuesday. Photo / AP
The fire in Notre-Dame Cathedral began about 4.50am NZT on Tuesday. Photo / AP

- AP