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Designer and singer Victoria Beckham on Thursday took on a new role as a UN ambassador on AIDS, saying she was transformed by a visit to South Africa.

Beckham said that she had long been interested in charity but had previously taken a supportive role in work led by her husband, British football icon David Beckham, as well as fellow singers Elton John and Annie Lennox.

But Beckham, who turned 40 this year and has four children herself, said she felt a new responsibility after traveling several month ago to Cape Town and meeting mothers living with HIV.

"South Africa was a huge turning point for me. And I don't know, why has it taken for me to get to 40 years old to realize, I need to stand up? I need to use what I have to make a difference," she said.

"For some reason, people will listen to what I have to say, so I'm going to speak on behalf of the incredible women" who have HIV and are caring for children, she said.

"I'm not going to sit here and pretend to know everything at this stage -- I don't -- but I want to learn. I want to help," she said next to a beaming Michel Sidibe, director of the UNAIDS agency.

Beckham will help in campaigning to promote an end to maternal transmission of HIV. She also recently auctioned part of her wardrobe to raise money for the charity mothers2mothers, which works on the issue.