GENEVA (Reuters) - Spreading ethnic violence is driving more people from their homes in the Democratic Republic of Congo where the humanitarian situation is “dramatically deteriorating”, the United Nations said on Monday.

Some 100,000 people were uprooted last week alone, bringing the ranks of displaced in the central Kasai region to nearly 1.3 million, it said. The total number of displaced throughout Congo has more than doubled to 3.7 million since Aug 2016.

“This very acute crisis in the DRC is not just expanding dramatically in terms of numbers but it’s also expanding in terms of geographical scope,” said Rein Paulsen, head of the U.N. Office for Humanitarian Affairs office in Congo.

“The fact that we are also now seeing an evolution of the conflict in the Kasais where inter-ethnic violence and conflict is becoming a dominant characteristic should be a deep, deep concern to all of us,” he told a news briefing.

The U.N. said last month it had documented 40 mass grave sites and killings of more than 400 people in Kasai, the focus of the fight against the Kamuina Nsapu militia, since August when security forces killed its leader. The militia has been fighting largely to avenge his death.

Paulsen cited fresh reports from U.N. staff of inter-ethnic fighting in the Kasais, including the Penda and Chokwe ethnic groups against the Luba and clashes between the Lunda and Luba.

In Manono, in the eastern province of Tanganyika, more than 140 villages have been reportedly burned down in a separate conflict between the pygmy population and Bantu ethnic groups, causing forced displacement, he said.

Across the vast Central African nation, an estimated 1.9 million children under five are severely acutely malnourished, a condition which could kill them or leave them with lifelong damage, Paulsen said.

The United Nations has received just 19 percent of the $812.5 million sought in the humanitarian appeal for Congo this year, he said.

The world body said last month it was horrified by a video screened by the government that appeared to show the brutal killing of two U.N. investigators.

Congo’s government said the film showed members of an anti-government militia carrying out the act although that has not been confirmed by either the U.N. or independent analysts.

Paulsen said that government workers - education inspectors and local transportation staff - had been “killed and beheaded” in the Kasais in the past week despite tighter security.