The damage to Notre-Dame cathedral is seen by drone

Firefighters lost valuable time in reaching the blaze at Notre Dame after a computer glitch sent investigators to the wrong part of the cathedral, according to French reports.

An initial fire alarm sounded at 6.20pm local time but after failing to find a blaze, security services at the landmark dubbed it a false alarm, according to sources cited by Le Parisien.

At 6.43pm, almost 25 minutes later, a second alarm went off. Only when they returned to the upper area of the edifice did they call in firefighters, after coming across three-metre flames at the base of the spire.

According to Le Parisien, the computer glitch had caused the alarm to signal the wrong location.

The reports came as the fund set up to help rebuild Notre-Dame passed one billion euros. Emmanuel Macron has pledged to restore the fire-ravaged cathedral in five years.

Donations from worshippers and wealthy French billionaires and corporations have seen hundreds of millions of euros raised since the cathedral was engulfed by flames on Monday.

Contributors include Apple and magnates who own L'Oreal, Chanel and Dior, as well as Catholics and others from around France and the world.

French television personality Stéphane Bern told broadcaster France-Info today: "We are at about 900 million euros and the one billion mark will be exceeded today."

It came after Mr Macron on Tuesday night set a five-year deadline to restore the 12th-century landmark. The president is holding a special cabinet meeting today dedicated to the disaster at Notre-Dame.

"We will rebuild the cathedral even more beautifully and I want it to be finished within five years," he said in the speech from the presidential palace. "We can do it."

Authorities consider the fire an accident.

Pope Francis today thanked Paris firefighters who risked their lives to save the Notre-Dame Cathedral on behalf of the Catholic Church.

During a weekly general audience in Saint Peter's Square, he said: "The gratitude of the whole Church goes to all those who did everything they could, even risking their lives, to save the Basilica."

View photos The damage to Notre-Dame cathedral is seen by drone More

Rebuilding in time for Olympics

Macron's announcement of a five-year restoration timeframe indicates he wants the reconstruction to be completed by the time Paris hosts the Olympic Games in 2024.

"We will rebuild the cathedral even more beautifully and I want it to be finished within five years," he said in the speech from the presidential palace. "We can do it."

Macron said that the dramatic fire had brought out the best in a country riven with divisions and since November shaken by sometimes violent protests against his rule.

It had shown that "our history never stops and that we will always have trials to overcome," he added.

Structural 'weaknesses'

Images from inside the cathedral showed its immense walls standing proud, with statues still in place and a gleaming golden cross above the altar.

However the floor was covered in rubble and scorched beams from the fallen roof and water while parts of the vaulting at the top of the cathedral had collapsed.

Junior interior minister Laurent Nunez told reporters that work to secure the structure would continue into Thursday.

He said the building had been saved within a critical time window of 15-30 minutes by a team of 400 firefighters who worked flat out through the night.

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