About 1,000 people have been killed this month in the country.
Amy Martin, head of the U.N. Office for the Coordination of Humanitarian Affairs in Bangui, said there was heavy weapons fire break out on Monday in clashes between government and Christian militias.
He told Reuters on Monday in Bangui that there was heavy weapons fire north of Bangui for a few hours and several neighbourhoods were affected.
French and African troops have struggled to contain violence between Muslim Seleka rebels and Christian militias that has already killed 1,000 people this month and displaced hundreds of thousands.
Reuters reports that witness in the capital reported shell explosions and mortar fire, adding that it had stopped by late morning.
Heavy arms fire was reported in Bangui during a two-day spike in violence which began on December 5 but reports of shooting in recent days has been limited to sporadic small arms fire.
Guy-Simplice Kodegue, the spokesperson for interim President Michel Djotodia, said the fresh fighting was between government forces and members of the Christian militia, known as anti-balaka after the local Sango language word for machete.
He did not say whether there had been any casualties.
A witness who preferred anonymity said a group of 40 men armed with rifles marched through northern Bangui on Monday, in spite of French-led efforts to disarm the population.
The country’s Christian majority has complained of waves of looting and killing by Djotodia’s loose band of militias who seized power in March with the aid of fighters from Chad and Sudan.
Violence intensified in early December after Christian militia launched reprisal attacks on Seleka forces, raising fears of generalised conflict in the country.
The number of internally displaced has swollen with the mounting violence and over 100,000 are sheltering in a makeshift camp at Bangui airport.