* UNICEF estimates up to 6,000 child soldiers in conflict
* World powers scramble to stem spiralling crisis
* France, U.N. officials say country risks genocide
By Stephanie Nebehay
GENEVA, Nov 22 (Reuters) - The number of child soldiers in Central African Republic has more than doubled to as many as 6,000 in recent months as self-defence militia have sprung up to counter waves of attacks by former rebels, the United Nations said on Friday.
World powers, led by France, are scrambling to contain a crisis that Paris and U.N. officials have warned could lead to genocide in Central African Republic, which slipped into chaos after rebels ousted the president in March.
Some 400,000 people have fled their homes since the uprising and the tit-for-tat killings that followed.
UNICEF, the U.N.'s children's agency, warned of a complex crisis that has seen local communities set up self-defence groups to halt abuses by the northern, mainly Muslim Seleka rebel alliance that swept to power.
This has led to a spike in the number of children involved.
"It has increased a lot. We can estimate this number at between 5,000 (and) 6,000 ... with the armed groups," Souleymane Diabate, UNICEF's resident representative in Central African Republic, told a briefing in Geneva.
UNICEF said in April, a month after Seleka's uprising, that more than 2,000 boys and girls were working as child soldiers.
That number swelled to 3,500 with the rebellion and surged again amid mounting violence, aid officials said, adding that the children were associated both with Seleka forces and the militias fighting them.
There is a 2,500-strong regional peacekeeping force in the landlocked country that is home to 4.6 million. Rebel chief and interim president Michel Djotodia has officially disbanded Seleka, many of whom are fighters from nearby Chad and Sudan.
But he has little control over his country and violence is increasingly pitting the mainly Muslim rebels against largely Christian militia. Christians make up half the population and Muslims 15 percent, the CIA World Factbook says.
French defence and foreign ministry officials began a tour of central Africa on Friday to discuss the crisis, a day after Paris said the United Nations would give France and African forces permission to intervene.
No plan for an intervention has been made public.
France has about 400 troops in Central African Republic, mainly protecting the airport and French assets in the capital Bangui, but French diplomatic sources said that Paris would consider ramping up numbers to between 700 and 1,200 if needed.
Warning that the Bangui government had neither the will nor the ability to stem the violence, the United States has pledged $40 million to support the African force, which is due to receive reinforcements next month.
However, diplomats said the mission was still hamstrung by delays and a lack of troops.
"If the international community doesn't stand up and do something, I think we will probably see a lot of killing going on there," said UNICEF's Diabate.
He added that the situation had been muddied by the local militia being "infiltrated" by former members of ousted president Francois Bozize's presidential guard.
Bozize, currently believed to be in Kenya, has said he is seeking to return to power.
Central African Republic is rich in gold, diamonds and uranium, but decades of instability and the spillover from conflicts in its larger neighbours have kept it mired in crises. (Writing by David Lewis; Editing by Emma Farge and Gareth Jones)