The United Nations has said that a militia group has killed 27 Muslims in the Central African Republic, in an attack underscoring the difficulties faced by French troops in stabilising their former colony.

The Christian militia, known as anti-Balaka, carried out the killings on Thursday in Bohong, a village about 75km from the town of Bouar.

The UN Human Rights office said that "the situation is also tense in several towns, including Bouca, Bossangoa and Bozoum, where a vicious cycle of attacks and reprisals continues."

Mostly Muslim Seleka rebels seized power of the country in March, ousting President Francois Bozize.

Since taking power they have conducted a string of abuses, prompting the creation of Christian defence groups, which in turn have deepened inter-religious conflict.

Christian militia and gunmen loyal to Mr Bozize attacked the capital last week, triggering fresh killings and reprisals. More than 500 people died and 100,000 were displaced from their homes in the capital, Bangui alone.

French troops, who now number 1,600 in the country, have restored some order to Bangui and begun disarming gunmen as well as moving out to other towns.

However the killings in Bohong point to the scale of the task involved in restoring order to a country the size of France.

"We condemn any attack on places of worship and on religious freedom, and urge all communities to exercise restraint," the UN Human Rights Council said in a briefing note.

Several people have also died in clashes in the Miskine neighbourhood of northwest Bangui overnight, a sign that the capital itself remains unstable.

Residents in Miskine said it was a Seleka stronghold and urged the French army and African peackeepers to step up their intelligence operations in a bid to bring calm.

There was no immediate comment from the French army.

Prime Minister Nicolas Tiangaye confirmed he would not stand at the next elections in accordance with a political accord signed in January.

"We will set up, in the next few days, the national transition authority. This structure, which is independent, is empowered to prepare and organise elections," he said in an interview on France 24.

France wants elections brought forward to next year, putting an end to the interim period originally scheduled to run into 2015

Meanwhile, the African Union has authorised increasing an African force being deployed to the country to 6,000 troops from 2,500,