A huge fire has broken out at the National Museum of Brazil in Rio de Janeiro, the oldest scientific institution in the country.Firefighters were tackling the blaze at the building, which contains more than 20 million items in its collection, the BBC reported on Monday.Brazil's President Michel Temer tweeted that it was a "sad day for all Brazilians", adding: "The value of our history cannot be measured by the damage to the building."It is not yet clear if anyone has been injured in Sunday's fire. The cause of the fire is not yet known.The museum, which once served as the residence for the Portuguese royal family, celebrated its 200-year anniversary earlier in 2018.Images broadcast on Brazilian television showed how the fire, which is believed to have started after the facility had closed for the day, had spread throughout the building, the BBC said.People reported on social media that the museum was completely consumed by the blaze. It housed mummies, meteorites, insects, and fossils amongst other things. "So sad to see history in flames".In an interview with Brazil's TV Globo, the museum's director said it was a "cultural tragedy".The museum houses thousands of items related to the history of Brazil and other countries including Egyptian artefacts, according to its website.Its natural history collection includes important dinosaur bones and a 12,000-year-old human skeleton of a woman -- the oldest ever found in the Americas.Employees had reportedly previously expressed concern about funding cuts and the dilapidated state of the building.