GEORGETOWN, Guyana -- At least 19 youths were killed and several injured in a school dormitory blaze in Guyana, the tiny South American country's fire brigade said on Monday. It is not yet known how the fire started Sunday in a dormitory housing girls aged 11-12 and 16-17, a person who helped the emergency services said under condition of anonymity. The building was entirely gutted by the inferno. "Fourteen youths died at the scene, while five died at the Mahdia District Hospital," said the fire department in a statement. The government had previously said 20 people died in the blaze at the Mahdia Secondary School in central Guyana. Guyana, with a population of 800,000, is the only English-speaking country of South America. It is a former Dutch and British colony which recently discovered it holds the world's largest per capita oil reserves. Among the poorest nations in South America, it hopes the discovery will help spur development. The country also boasts the second-highest percentage of forest cover on earth. After the weekend tragedy more than a dozen children received hospital treatment locally while six serious cases were airlifted to the capital Georgetown. "Two children remain in critical condition, while four are nursing severe injuries as a result of the incident," added the fire brigade. "This is a major disaster. It is horrible, it is painful," President Irfaan Ali said on Sunday night. Ali said he had ordered arrangements to be made in the capital Georgetown's two major hospitals "so that every single child who requires attention be given the best possible opportunity to get that attention." Private and military planes have been sent to Mahdia, located about 200 kilometers (124 miles) south of Georgetown, as the region is affected by heavy rains. "Five planes have already taken off to Mahdia to support the regional health officials with additional medical supplies and medivacs," the government statement said. At least one plane with three evacuees arrived back in Georgetown, according to an AFP journalist. The government said officials were supporting efforts at Ogle airport in the capital to "receive the critical patients and coordinate an emergency plan of action." "A full-scale medical emergency action plan has been launched," it added. Natasha Singh-Lewis, an opposition MP, called for an investigation into the fire's cause. "We need to understand how this most horrific and deadly incident occurred and take all necessary measures to prevent such a tragedy from happening again," she said.