French President Emmanuel Macron has told workers who revived Paris' iconic, fire-ravaged Notre Dame cathedral, that they "achieved what was thought impossible."

French President Emmanuel Macron praised on Friday the more than 1,000 craftspeople who helped rebuild Paris' Notre-Dame Cathedral in what he called "the project of the century", five-and-a-half years after fire gutted the Gothic masterpiece.

You’ve achieved what was said to be impossible,” the French president told workers at the Paris monument, which will reopen after the 2019 fire.