LONDON, Nov 1 (Thomson Reuters Foundation) - At least 15 million teenage girls worldwide have been forced into sex - often by partners, relatives or friends - yet only one in 100 sought help, the United Nations said on Wednesday.

Cameroon had the highest rate of sexual violence, with one in six teenage girls experiencing forced sex, the U.N.'s children's agency (UNICEF) said in a report which examined data from more than 40 countries.

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"This idea of women being at the disposal of men is a big factor driving the experience of sexual violence of girls," report author Claudia Cappa told the Thomson Reuters Foundation.

In the majority of cases, the abuser was known to the victim - with acts of sexual violence carried out by husbands, boyfriends, family members, friends and classmates.

UNICEF said widespread sexual violence against teenage girls could hinder global progress towards achieving the U.N.'s Sustainable Development Goals (SDGs) - a plan to end poverty, hunger, achieve gender equality and protect the planet by 2030.

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The number of girls who have been forced to have sex is likely to be far higher than 15 million as many are reluctant to come forward and data is lacking in many countries, it said.

Abuses ranged from child sexual exploitation in the Dominican Republic's tourism industry to online sexual abuse in the Philippines. The report also highlighted projects to combat violence, including self-defence classes in schools in Malawi.

Better laws to protect children and more support from social services are vital to bring about change, UNICEF said.

"What has proved to be particularly successful has been working with governments to develop national action plans that try to bring together different sectors, such as education and the justice system," said Cappa.

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(Reporting by Lee Mannion @leemannion, Editing by Kieran Guilbert and Katy Migiro. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit http://news.trust.org)