UNITED NATIONS, April 15 (Xinhua) -- As Notre Dame Cathedral burned on Monday, United Nations Secretary-General Antonio Guterres expressed his "horror" at the images of the fire, shown live around the world.

"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," the UN chief said on Twitter.

Audrey Azoulay, head of UNESCO -- the Paris-based UN agency for education, science and culture -- also expressed her "deep emotion" on social media.

Azoulay wrote that UNESCO is closely monitoring the situation and is standing by the French people's side to "safeguard and restore this invaluable heritage."

The UN elevated Notre Dame, widely considered to be one of the finest examples of French gothic architecture, to world heritage status in 1991.