UN experts charged with monitoring the arms embargo in the Central African Republic cited on Wednesday an "influx of foreign fighters" into the country ahead of elections later this year.
The annual report said a "series of clashes was... fed by arrivals of foreign fighters and weaponry, mainly from the Sudan."
"Regional arms trafficking also continued through other routes," it added, calling for the Central African Republic, Chad and Sudan to "strengthen their efforts to combat the escalation in the flow of arms and foreign fighters" into the country.
One of the world's poorest and most unstable nations, CAR spiralled into bloodshed after former president Francois Bozize was overthrown in 2013.
Fighting has since forced nearly a quarter of the country's 4.5 million people to flee their homes, and rival militia groups control most of the country.
A presidential election is due in December 2020.
"The prospect of elections represented an additional incentive for armed groups to maintain and extend their control over territory," said the UN report which will serve as the basis for renewing the arms embargo, in place since 2013.
Two predominantly Fulani armed groups seek to extend their territory to maximize taxation of seasonal herders, it added.
"The Union pour la paix en Centrafrique (UPC), led by Ali Darassa, and Retour, reclamation et rehabilitation (3R), led by Abbas Sidiki, expanded their areas of control," it said.