(CNN) Priceless artifacts spanning 11,000 years went up in flames as an inferno swallowed Brazil's National Museum.

More than 20 million pieces of history, including Egyptian mummies and historic artwork, may have been destroyed.

The cause of the fire is not yet known.

"The loss of the National Museum's collection is insurmountable for Brazil," President Michel Temer tweeted.

No one knows what sparked the blaze around 7:30 p.m. Sunday. Firefighters worked through the night to battle the flames, but a museum official said the damage is already "irreparable."

"Very little will be left," preservation director Joao Carlos Nara told Agencia Brasil. "We will have to wait until the firefighters have completed their work here in order to really assess the dimension of it all."

Read More