Notre Dame cathedral blaze 'so horrible': Trump

US President Donald Trump on Monday called the blaze engulfing Notre-Dame cathedral in Paris "horrible" and suggested the deployment of flying water tankers.

"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.

Fire spread rapidly through a major portion of the magnificent Gothic cathedral at the heart of Paris, causing a spire to collapse and raising fears over the building The cause of the blaze was not immediately confirmed, but the cathedral had been undergoing intense restoration work.

Later, before making a speech on the economy in Minnesota, Trump said the television images he'd watched during his flight were "a terrible sight to behold." "It was burning at a level you rarely see a fire burn," he told the crowd.

He added: "It's a part of our culture, it's a part of our lives.