The fire that broke out at Notre-Dame Cathedral in Paris Monday evening has generated countless proposals worldwide on how to put it out, including one from US President Donald Trump.

But at least one expert found it “unrealistic.”

Fire engulfs Notre Dame cathedral. (Photo: CNN)

"Perhaps flying water tankers could be used to put it out. Must act quickly!" Trump said in a tweet.

“No pilot could drop water "exactly in that one spot moving several hundred miles an hour over it,” said Glenn Corbett, associate professor of fire science at New York's John Jay College of Criminal Justice.

Corbett also ruled out the use of helicopters. ”One of the issues you've got here, is thermal updraft, that's a chimney effectively. You can't fly a helicopter in hot air. The air is so thin."

French civil security agency Securite Civile also said in an apparent reference to Trump's suggestion that any aerial water dumping could "weaken the structure of Notre-Dame and result in collateral damage to the buildings in the vicinity."

The blaze collapsed the cathedral’s spire and destroying much of the roof, but the two bell towers and main edifice were spared. 

Most of its artwork and religious relics were said to have also been rescued, as around 400 firefighters were mobilized for the mission, pumping water from the Seine and flying drones to survey the damage.

Notre Dame's wooden roof beams and soaring Gothic architecture made the fire more difficult to control.

(Compiled by Bai Yuanqi)