* Sectarian killings spreading across the country
* Access for humanitarian workers hindered by security
* Deployment of AU peacekeepers slow, U.N. force not agreed (Adds ambassador change, diplomats)
By Joe Bavier
ABIDJAN, Nov 28 (Reuters) - Central African Republic needs up to four times more peacekeepers than are now deployed to quell a worsening sectarian conflict and provide security for aid workers, the European Union's top humanitarian official said.
The country has descended into chaos since the Seleka coalition of rebels, many of them from neighbouring Chad and Sudan, ousted President Francois Bozize in March.
France is preparing to boost its force in its anarchic former colony to at least 1,000 soldiers once a U.N. resolution is passed next week to improve security until a 3,600-strong African Union (AU) force is operational.
Paris, which already has around 400 troops based at the airport in the capital Bangui, has already started beefing up personnel and equipment in the country, diplomatic sources said.
Two sources also said France's ambassador to Central African Republic was being replaced, replicating a change of its envoy in Mali two months after French troops launched a mission there earlier this year to oust al Qaeda-linked militants.
France's foreign ministry was not immediately available for comment.
Around 2,500 regional peacekeepers deployed in the country are to be brought into the AU force.
"Clearly what needs to be done is beefing up of peacekeeping forces. Tripling or quadrupling what is there," EU aid chief Kristalina Georgieva said, warning they face a twin risk of a Somalia-like state collapse and potential genocide.
"Unless there is an immediate, significant change in security conditions, these two risks can deepen so much that we have a tragedy on our hands. And we'll look back and say 'why didn't we act sooner'," she said.
Some 460,000 people, a tenth of the population, have fled the sectarian violence since the takeover by the mainly Muslim Seleka rebels, whose numbers Georgieva said had grown from around 5,000 fighters to some 20,000 today.
Fearing that tit-for-tat killings could escalate into full-blown war between the Christian majority and Muslims, who represent around 15 percent of the population, world powers are scrambling into action.
U.N. Secretary-General Ban Ki-moon this month ordered his officials to start preparing for the likely deployment of a U.N. peacekeeping mission. But African leaders want to give the AU force time to try to stabilise the situation.
"There has to be a commitment now - not in one month, not in three months, now - to strengthen security," Georgieva told Reuters late on Wednesday during a visit to Ivory Coast.
She said rapidly deteriorating security was already hampering humanitarian assistance to the country of 4.6 million people and aid agencies worried that their workers could soon become targets of militia fighters.
Two local employees of the French humanitarian organisation ACTED were robbed and murdered in the country in September.
The French-drafted U.N. resolution would give a six-month mandate for French troops and the African-led International Support Mission (MISCA) to restore order, protect civilians and rebuild state authority.
"French troops will secure the main arteries and secondary roads," said a French diplomatic source. "It's completely feasible. This is neither al Qaeda in Mali nor al Shabaab in Somalia. I wouldn't say the Seleka is a flock of sparrows, but it should disband pretty quickly." (Additional reporting by John Irish in Paris; Editing by Bate Felix and Gareth Jones)