UN chief Antonio Guterres expressed horror and sadness at the devastation of the famed Notre-Dame Cathedral in Paris as fire tore through the 12th-century symbol of faith and architectural brilliance, days just before the world will celebrate Easter. The colossal fire swept through the famed cathedral in central Paris on Monday, causing a spire to collapse and threatening to destroy the entire masterpiece and its precious artworks. The fire, which began in the early evening, sent flames and huge clouds of grey smoke billowing into the Paris sky. "Horrified by the pictures coming from Paris with the fire engulfing Notre-Dame Cathedral - a unique example of world heritage that has stood tall since the 14th century. My thoughts are with the people and government of France," Secretary-General Guterres said in a tweet as images began to flash on television and social media of a burning Notre- Dame Cathedral. Audrey Azoulay, head of UNESCO \u2013 the Paris-based UN agency for education, science and culture \u2013 also expressed her "deep emotion in the face of this dramatic fire at the cathedral." Azoulay said that UNESCO is closely monitoring the situation and is standing by the French people's side to "safeguard and restore this invaluable heritage." The Organisation elevated Notre-Dame, widely considered to be one of the finest examples of French gothic architecture, to world heritage status in 1991. Construction of the cathedral began in 1163, on the central Parisian island in the middle of the Seine, known as Ile de la Cite, during the reign of King Louis VII, and it was finally completed in 1345. The President of the General Assembly Maria Fernanda Espinosa said in a tweet that she is "deeply saddened\u201d by the images of the burning Notre Dame Cathedral. One of the jewels of French and European cultural heritage, the cathedral was inscribed as UNESCO World Heritage site in 1991. "I stand with the people and government of France," she said. The devastation came days just before the world will celebrate Easter on April 21. The cause of the fire was not immediately known. Around 400 firefighters battled the blaze for nearly five hours. According to Paris City Hall, the fire started shortly before 7pm Paris time Monday. About 11 to 13 million visitors visit Notre Dame each year. The cathedral was undergoing much-needed renovations. While the cathedral's iconic spire and roof collapsed in the fire, the main structure of the Gothic cathedral was saved from total destruction.