* U.N. aid flown in after complaints of slow response
* French minister visits amid clashes in Bangui
* PM confirms he will not stand at next elections (Recasts with U.N. warning, adds French minister, AU troops, UN aid supplies)
By Emmanuel Braun
BANGUI, Dec 13 (Reuters) - The United Nations on Friday warned groups carrying out atrocities in the Central African Republic the world was watching and would hold them to account, after the killings of hundreds of people, mainly civilians.
The warning from U.N. Secretary-General Ban Ki-moon came a day after a militia killed 27 Muslims in a village outside the capital, Bangui, underscoring the challenge international troops face stabilising the country.
Confronted with a deepening humanitarian crisis and criticism from some aid workers that it was reacting too slowly, the United Nations flew in 77 tonnes of relief supplies, the largest airlift since fighting last week.
Central African Republic has been paralysed by cycles of violence since mainly Muslim Seleka rebels ousted President Francois Bozize in March. Their months in power have been marked by killings, looting and other abuses, leading to the emergence of Christian militia opposed to them.
These militia and gunmen loyal to Bozize attacked the capital last week, triggering fresh killings and reprisals that have deepened inter-religious conflict. More than 500 people died and 100,000 were displaced in the capital alone.
"Too many people are scared and the country is on the brink of ruin ... The bloodshed must stop," Ban said in a radio address to the nation.
"I have a clear message to all who would commit atrocities and crimes against humanity. The world is watching. You will be held to account," he added. The International Criminal Court has said all parties could be investigated.
A U.N.-authorised French peacekeeping mission has restored a degree of calm to the capital but conflict has spread and the U.N. Human Rights office said the Christian militia, known as anti-balaka, killed 27 Muslims on Thursday in Bohong, a village about 75 km (45 miles) from the western town of Bouar.
"The situation is also tense in several towns, including Bouca, Bossangoa and Bozoum, where a vicious cycle of attacks and reprisals continues," it said in an email.
French troops, who number 1,600 in the country, say disarmament in the capital is drawing to an end and troops have begun disarming gunmen in other towns. But the Bohong killings point to the scale of the task in a country the size of France.
French Defence Minister Jean-Yves Le Drian said Paris had been surprised by the scale of the violence in the capital.
"We thought the risk would have been bigger up country than in Bangui but in fact it was the other way round," he said during a visit on Friday, adding that deployments outside Bangui would gradually pick up once the capital was secured.
As international efforts to tackle the crisis accelerate, the African Union has authorised increasing its force in the country to 6,000 troops from 2,500.
The country is rich in diamonds, gold and uranium but has seen little stability and France has intervened there more since independence in 1960 than in any other former colonies.
Several people died in clashes in the Miskine neighbourhood of northwest Bangui overnight and Friday morning, according to witnesses, a sign the capital itself remains unstable.
The fighting started when Christians on Thursday looted the motorbike shop of a man linked to the Seleka, and escalated into reprisal killings. French troops, backed by a helicopter, restored calm on Friday, they said.
"The tension is still high in the neighbourhood despite the presence of the French," said Chancella Cazalima, a student.
Residents in Miskine said it was a Seleka stronghold and urged the French army and African peacekeepers to step up their intelligence operations in a bid to bring calm.
There was no immediate comment from the French army.
UNICEF said on Friday it had flown in tonnes of supplies, including blankets, soap, jerry cans and medicines. "This new arrival of emergency supplies is critical to prevent diseases, especially among the most vulnerable children and women."
French medical charity Medecins Sans Frontieres had on Thursday accused U.N. agencies of failing to mobilise resources quickly enough to the crisis, which has forced 500,000 people from their homes over the last year.
Prime Minister Nicolas Tiangaye confirmed he would not stand at the next elections in accordance with a political accord signed in January. An independent body to prepare elections would be set up in days, he added.
France wants elections brought forward to next year, putting an end to the interim period originally scheduled to run into 2015. (Additional reporting by Nicholas Vincour in Paris and Stephanie Nebehay in Geneva; Writing by David Lewis and Matthew Mpoke Bigg; Editing by Andrew Roche)