She pledged to speak out for millions of children and young people whose voices had been silenced, shine a light on issues that vulnerable children suffer around the world and make sure children know their rights and empower them.

Brown said she would use her platform, which includes more than 18 million followers on Instagram. She was also the youngest person to make Time magazine's list of the world's 100 most influential figures.

"In becoming a UNICEF goodwill ambassador, I share the title with a hero of mine - the late, great Audrey Hepburn, who once said: 'As you grow older you will discover that you have two hands, one for helping others and one for helping yourself'," Brown told a news conference announcing her appointment.

"That's exactly what I intend to do," she said.

Reuters