Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 24, 2019 shows that a statue is transferred from the burnt Notre Dame Cathedral in Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year. (Photos: Xinhua)

Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

The Notre Dame Cathedral is seen in Paris France, April 8, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on Oct. 17, 2019 shows people watching the Notre Dame Cathedral under repair in Paris, France. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 24, 2019 shows temporary tarpaulins protecting the Notre Dame Cathedral from rain damage in Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year.

Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 15, 2019 shows the Notre Dame Cathedral on fire in central Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 15, 2019 shows the Notre Dame Cathedral on fire in Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year.

Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 24, 2019 shows workers on top of the burnt the Notre Dame Cathedral in Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year.

Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 16, 2019 shows the Notre Dame Cathedral after a fire in Paris, France. The cathedral in central Paris caught fire on April 15 last year.

File photos taken on April 15, 2019 shows firefighters try to extinguish the fire on the Notre Dame Cathedral in Paris, capital of France. The cathedral in central Paris caught fire on April 15 last year.

Restoration work on the Notre Dame Cathedral is at a standstill during the containment period caused by COVID-19 outbreak, in Paris, France, April 15, 2020. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on April 16, 2019 shows a woman watching the Notre Dame Cathedral after a fire in Paris, France. The cathedral in central Paris caught fire on April 15 last year.

File photo taken on Aug. 16, 2019 shows people walking past the Notre Dame Cathedral under repairs in Paris, France. The cathedral in central Paris caught fire on April 15 last year.