Rebels in the eastern Democratic Republic of Congo have killed five Congolese aid workers and two other civilians, a UN agency in the country said.
A statement released on Thursday said the attack targeted a vehicle transporting members of a local educational group south of Sange, in South-Kivu province, on Tuesday.
The UN's Office for Co-ordination of Humanitarian Affairs (OCHA) said it was the deadliest attack on aid workers ever recorded in the country and had left colleagues "very shocked".
"It strengthens the feeling of insecurity, which has reigned for several months in humanitarian organisations in the east of the country," it said.
Local officials blamed the attack on members of the Congolese Mai Mai Yakatumba militia and allied rebels of Burundi's National Liberation Front.
"We are in the process of taking steps to secure the area. It's each day that Yakatumba and its allies, the FNL and FDLR attack civilian vehicles, boats, even entire villages. This must stop," local administrator Selestin Kalume Mwanshima told the Reuters news agency by telephone.
FNL is a Burundian rebel group based in Congo that was blamed for last month's attack on a bar in Burundi in which more than 30 people were killed.
The FDLR is a Rwandan rebel group operating in Congo accused of carrying out numerous atrocities against civilians, including mass rapes.
Both local and foreign rebel groups in the Democratic Republic of Congo wage war on each other and the civilian population.
OCHA said that nearly 25 incidents involving aid workers had been recorded since August in Nord-Kivu province and some 15 in neighbouring Sud-Kivu.
"These incidents range from hold-ups to hostage-taking and the hijacking of humanitarian vehicles to transport military equipment," it said.