Tens of thousands of people have been forced to flee "unprecedented levels of violence" in the Central African Republic, the medical charity Medecins Sans Frontieres (MSF) has said.
It said its workers had seen appalling scenes of murder in a wave of attacks.
Entire villages had been burnt to the ground and people have been treated for gunshot and machete wounds, MSF said.
The mineral-rich but underdeveloped country has been in a state of chaos since rebels seized power in March.
A rebel alliance known as Seleka, commanded by Michel Djotodia who is now president, ousted Francois Bozize from office.
Last month, Mr Djotodia formally disbanded the rebels and integrated many fighters into the national army.
However, rebels linked to Seleka have continued to launch attacks on scores of villages, prompting the emergence of local civilian protection groups.
MSF said it was one of the few organisations working in the conflict-affected areas of the north-west of the country.
"MSF teams are horrified by what they're witnessing, including the execution of a healthcare worker, multiple violent attacks on humanitarian staff, burned villages, and appalling scenes of murder," MSF surgeon Erna Rijinierse said in a statement.
Last month MSF teams treated more than 60 people, including children, in the town Bossangoa, 250km (155 miles) north-west of the capital, Bangui, most of them with gunshot and machete wounds,
The medical charity says more than 30,000 people in Bossangoa have been displaced by the fighting between rebels and the vigilante groups.
There were also numerous accounts of attacks characterised by religious divisions, it said.
Most of the displaced in Bossangoa are sheltering in the town's Catholic Mission, far exceeding its capacity, about 1,200 people are in a hospital, "effectively turning half of the building into a makeshift camp", MSF says.
A further 1,000 are seeking shelter next to an airstrip and 100 others have gathered in a school, the charity says.
They are living in precarious conditions - vulnerable to malaria, the number one killer in CAR, it adds.
Last week the UN Security Council approved a resolution paving the way for the deployment of a UN peacekeeping force in the country.
On Sunday, France's Foreign Minister Laurent Fabius said more French troops would be sent to CAR in an effort to end the chaos.
The former colonial power already has about 400 troops based at the airport in Bangui.
The African Union is in the process of deploying a 3-600-strong peacekeeping mission to the country - incorporating a regional force already on the ground.
CAR has huge deposits of precious minerals but has been plagued by chronic instability since independence in 1960.