Civilians in both the Central African Republic (CAR) and south Sudan have come under from the Lord's Resistance Army (LRA), aid agencies have said.
The Ugandan rebel group is said to have killed people in several CAR towns, forcing survivors to flee.
The south Sudanese town of Ezo was also attacked recently by the LRA, which has a history of rape and killings.
The LRA uses its base in the Democratic Republic of Congo to launch incursions across poorly defended borders.
A number of people died in the latest attacks, according to United Nations workers and local aid agencies.
It is also being reported that some LRA members were killed as CAR townspeople tried to defend themselves.
The World Food Programme has already provided food aid to 60,000 people fleeing LRA attacks this year.
The rebel group's leaders, notorious for capturing children and using them as fighters, are wanted by the International Criminal Court.
Earlier in 2009 Ugandan troops carried out an operation to pursue the Lord's Resistance Army into north-east Democratic Republic of Congo, intending to put a stop to attacks into neighbouring countries.
But the landscape of dense jungle and swamps has made the group very hard to pin down.