One of the largest anthropology and natural history collections in the Americas was nearly totally destroyed in Sunday's fire in Rio de Janeiro.
It was not yet known what caused the fire, which started after the museum closed on Sunday.
He continued in a statement, "Two hundred years of work, investigation and knowledge have been lost".
Luiz Duarte, vice director of the museum, told Globo TV the institution had been neglected by successive federal governments, and that a 21.6 million real ($5.23 million) financing plan announced in June included, ironically, a plan to install modern fire protection equipment.
The National Museum, which is linked to the Federal University of Rio de Janeiro, has suffered from funding cuts.
Roberto Robadey, a spokesman for the Rio fire department, is quoted by the Associated Press news agency as saying that the hydrants closest to the museum were not working and that firefighters had to get water from a nearby lake.
"The museum had 0 mechanisms to help contain the fire".
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Culture Minister Sergio Sa Leitao said it was a "tragedy that could have been avoided" but a reconstruction effort would begin.
The palatial National Museum building used to be the home of a Portuguese royal family.
Kellner said that the institution had just recently secured approval for funds for a planned renovation of the museum, including an upgrade of the fire prevention system.
First built in 1818 as a residence for Portugal's royal family, the edifice also contained insects, mummies, paintings and dinosaur bones.
The museum houses thousands of items related to the history of Brazil and other countries including Egyptian artefacts, according to its website. "Collections that are over 100 years old", Cristiana Serejo, one of the museum's vice-directors, told the G1 news site.
Some Brazilians saw the fire as a metaphor for their country's traumas as it battles terrifying levels of violent crime and the effects of a recession that has left more than 12 million people unemployed.
But the fire quickly led to criticism over dilapidated infrastructure and budget deficits as Brazilians prepare to vote in national elections in October.