By Katy Migiro

NAIROBI, Dec 11 (Thomson Reuters Foundation) - Child marriage is on the rise in Ethiopia due to the worst drought in decades, the government and agencies said on Friday, as Oxfam warned of a "full-blown disaster" unless more than $1 billion in food aid is found for 10 million people.

Agencies predict the El Niño weather phenomenon will cause record levels of malnutrition in Africa's second most populous nation, famed for war- and drought-induced famine in 1984.

More than one in ten of Ethiopia's 92 million people, most of whom depend on rain-fed agriculture, are short of food.

"Children in assessed areas are showing distressed behaviour due to food shortages, increase in child marriage and family separation," Ethiopia's government and charities said in a 2016 humanitarian appeal on Friday.

"Women face risks of different forms of violence, including when eroded livelihoods forces them into survival sex."

Child marriage is a major issue in Ethiopia where 41 percent of girls are married before their 18th birthday, the campaign group Girls Not Brides says.

During droughts, families often marry girls off at earlier ages as this earns them dowry and increases the girls' chances of being fed by wealthier husbands.

Early marriage deprives girls of education and opportunities and increases the risk of death or serious childbirth injuries if they have babies before their bodies are ready. Child brides are also at greater risk of domestic and sexual violence.

Child labour and school dropouts are likely to rise as drought intensifies, the appeal said, and girls risk sexual assault walking long distances to fetch water.

"This is a serious drought and we expect the situation to get worse between January and March," Oxfam's humanitarian director Jane Cocking said in a statement.

"It's vital the international community commits funding now to stop this food crisis becoming a full-blown disaster."

Ethiopia has enjoyed strong growth over the last decade and the government has an impressive disaster management system.

But the scale of the current El Niño, a warming of the Pacific Ocean causing hunger across east and southern Africa, is "unprecedented", the United Nations said.

Agencies called on donors to respond quickly.

"The highest priority remains food - some $1.1 billion is urgently required," Gillian Mellsop, the head of the U.N. children's agency UNICEF in Ethiopia, said in a statement.

"This can prevent needless suffering and far more costly specialised nutrition interventions."

It is ten times cheaper to treat children at risk of malnutrition before they reach a critical state than to pay for life-saving therapeutic care, experts say. (Reporting by Katy Migiro; Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, corruption and climate change. Visit www.trust.org)