Clashes between an armed group and a self-defence group in southeastern Central African Republic have left at least 30 civilians dead, including six Red Cross volunteers who were attending a crisis meeting at a health facility, officials said on Wednesday.

The violence took place late last week in the town of Gambo, which is about 75 kilometres from Bangassou, a town that has been a flashpoint amid the upsurge of bloodshed in the long chaotic country.

"We are appalled by the news of the death of our fellow volunteers," said Antoine Mbao-Bogo, president of the Central African Red Cross. "We call on all parties to take steps to spare the civilian population, and to respect all humanitarian workers."

READ MORE: Why are civilians still being killed in the CAR?

The deaths marked the third such attack on the Red Cross this year, officials said. Many humanitarian workers have been blocked from doing their work, and fighters also have tried to kill wounded enemies in public health facilities.

The violence late last week also underscored the deepening conflict in and around Bangassou, where at least nine UN peacekeepers have been killed this year alone.

Observers warn that the sectarian violence that erupted in 2013 in Central African Republic's capital has now moved into the southeast, prompting warnings of a national conflict roaring back to life.

Gambo fighting blamed on Seleka fighters

Local authorities blamed the Gambo fighting on a faction of the mostly Muslim rebel group once known as Seleka - fighters who now call themselves the UPC. The fighters battled against a self-defence group that has arisen in the area in an effort to ward off the UPC and other armed fighters. At times the UPC fighters shot arrows to kill their victims, local authorities said.

Central African Republic currently has some 12,000 UN peacekeepers but none are in Gambo and none was present during the deadly clashes last week.

Souleymane Daouda, a UPC spokesman for the faction led by Ali Darassa, denied to the Associated Press agency that his fighters were responsible for the violence in the town of Gambo, saying his men were protecting civilians.

"A thousand people attacked the town of Gambo," Daouda said. "As our elements were there, they reacted by evacuating the civilian population first and then, on their return, they used arrows to defeat these armed bandits who massacre the population."

While UN peacekeepers were not there at the time of the violence, spokesman Vladimir Monteiro said reinforcements had been sent to the area to reassure residents.

The diamond-rich but poverty-stricken nation has been in crisis since late 2012, when violence broke out between Muslim and Christian armed groups. A peace deal signed by the government and 13 armed groups in June was followed by renewed fighting.

Renewed clashes in the Central African Republic are early-warning signs of genocide, the UN aid chief said on Monday, calling for more troops and police to beef up the UN peacekeeping mission in the strife-torn country.