Thirteen people have been killed in a town in the Central African Republic, a local doctor said on Sunday, as the toll from sectarian violence that has sparked fears of genocide continued to rise.

The clashes took place in Bria, in the centre of the country 450 kilometres from the capital Bangui, between a majority Muslim rebel group and a predominantly Christian armed group called the Anti-balaka.

"Thirteen bodies were found on Saturday after violent fighting between self-defence forces and parts of Abdoulaye Hissene's FPRC," Michel Ambapo told AFP news agency, referring respectively to the Anti-balaka and a faction of the rebel coalition known as the Seleka.

"At least 20 wounded were admitted to hospital, most of them combatants on both sides and several civilians," the doctor added.

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The total number of deaths in the city since fighting erupted several days earlier "is around 30," Ambapo said.

The UN's peacekeeping force, known as MINUSCA, did not report a toll.

'On the brink of catastrophe'

On Saturday, sources reported that fatal clashes had taken place in several areas of the country in previous days.

They included more than a dozen fatalities in the southeastern town of Zemio, Jean-Alain Zembi, a priest, told AFP.

Six aid groups wrote to the United Nations last Tuesday, describing the former French colony as "on the brink of catastrophe".

We "request your office take immediate action to prevent the country collapsing into another full-blown conflict", the letter said, adding that "at least 821 civilians have been killed since the start of the year".

One of the world's poorest nations, Central African Republic was pitched into a war between Muslim and Christian armed groups in 2013, unleashed when President Francois Bozize, a Christian, was overthrown by the Seleka.

The Seleka, in turn, were overthrown by a military intervention led by France, which was followed bloody reprisals by the mainly Christian Anti-balaka.

Groups on both sides are now fighting for control of natural resources, including gold and diamonds, as well as regional influence. Half a million people, in a country of 4.5 million, have fled.

On August 7, UN aid chief Stephen O'Brien warned that the situation was such that he saw "early warning signs of genocide".