United Nations, Dec 14 : UN Secretary-General Antonio Guterres has strongly condemned an attack on a secondary school in Nigeria's Katsina state and the reported disappearance of at least 333 students, his spokesman said. The Secretary-General called for the immediate and unconditional release of the abducted children and for their safe return to their families, Xinhua news agency quoted spokesman Stephane Dujarric, as saying in a statement on Sunday. Guterres reiterated that attacks on schools and other educational facilities constitute a grave violation of human rights. He urged the Nigerian authorities to bring those responsible for this act to justice, said the statement. The Secretary-General reaffirmed the solidarity and support of the UN to the government and people of Nigeria in their fight against terrorism, violent extremism and organised crime, the statement added. The attack on the Government Science Secondary School took place on the night of December 11. The boarding school located in the Kankara area of the state has a population of 839 students. "Based on the available record we have, we are still searching for 333 students through either the forest or their parents to ascertain the actual number that has been kidnapped," state Governor Aminu Masari said on Sunday. "We are still counting because more are coming out from the forest and we are calling through the numbers of those parents that have phone numbers to find out whether or not their children have gone back home. "We as a government are yet to be contacted by any group or person responsible for the kidnap of the students," he added. Masari has ordered the immediate closure of all boarding secondary schools in the state. President Muhammadu Buhari has condemned the attack, ordering the security agencies to nab the attackers to ensure that no student was harmed or goes missing. /IANS