Paris prosecutors say that while they are investigating the possibility of negligence, they currently have no reason to believe the fire was started deliberately.

The blaze ripped through the medieval cathedral on April 15, destroying the roof, toppling the spire, and almost bringing down the main bell towers and outer walls before firefighters brought it under control.

"If certain failings, which may explain the scale of the fire, have been brought to light, the investigations carried out to this date have not yet been able to determine the causes of the fire," a statement from Paris prosecutor Remy Heitz said.

"For now, there are no indications of a criminal origin," he added on Wednesday.

However, nothing had been ruled out, with an electrical fault and a cigarette that was not properly extinguished two of several possible causes being investigated, he said.

President Emmanuel Macron has set a target of five years for restoring Notre-Dame, which dates back to the 12th century and is one of Europe's most iconic landmarks.

Australian Associated Press