Sudanese rebels say they have launched a major offensive against Uganda's notorious Lord's Resistance Army.
The LRA rebels is known for its brutality
The Sudanese groups say they are retaliating for LRA atrocities committed in southern Sudan, from where they launch attacks on Uganda.
Uganda hopes the LRA will now be tied up fighting in Sudan and so will be less able to attack Uganda.
The LRA is accused of killing some 200 refugees in northern Uganda last month - the worst atrocity in its 17-year war.
The region has a complicated network of armed groups, which frequently form new alliances with each other and rival governments.
Sudan's main rebel group, the SPLA, is part of the new offensive against the LRA.
SPLA spokesman George Riek Machar told Reuters news agency that some 86 Ugandan rebels had been killed in a week of skirmishes in southern Sudan.
He said the LRA was being given supplies in Torit, a southern Sudanese town controlled by Sudanese government forces.
Uganda says the Sudan government is backing the LRA, however Khartoum has given the Ugandan army permission to pursue the LRA on Sudanese territory.
The SPLA is joined in its offensive by the Equatorial Defence Forces, which says the LRA has attacked its positions south of Juba, the main town in southern Sudan.
"They have been attacking soft spots like schools and churches and recently hacked four people to death including my father-in-law," said EDF leader Theopilous Ochang Lotti at a news conference in the SPLA offices in the Kenyan capital Nairobi.
He denied that the new offensive was being organised with the Ugandan army.
Following the massacre of refugees, Uganda's President Yoweri Museveni has set up camp near Lira, saying he is determined to wipe out the rebels - something he has repeatedly promised to do in the past.