Many LRA fighters are abducted children
Uganda's army has accused the Lord's Resistance Army rebels of hacking to death 45 civilians in a Catholic church in the Democratic Republic of Congo.
Capt Chris Magezi said the scene was "horrendous... dead bodies of mostly women and children cut in pieces". The attack happened on 26 December.
A rebel spokesman has denied responsibility for the killings, which follow a collapse in the peace process.
The UN says at least 189 people were killed in several attacks last week.
Some reports say more than 100 people were killed in the church alone.
The armies of Uganda, South Sudan and DR Congo carried out a joint offensive against the rebels in mid-December after LRA leader Joseph Kony again refused to sign a peace deal.
The LRA leader, who has lived in a jungle hideout in north-eastern DR Congo for the last few years, is wanted by the International Criminal Court for war crimes and crimes against humanity.
Uganda's government had been involved in lengthy peace negotiations with the LRA, hosted by the South Sudanese government.
But Mr Kony has demanded that arrest warrants for him and his associates be dropped before any agreement can be struck.
Meanwhile, the UN peacekeeping mission in DR Congo says one of its troops accidently shot and killed a Ugandan soldier in the nearby town of Dungu.
Aid officials requesting anonymity near Doruma, which is about 40km from the border with South Sudan, confirmed to Uganda's Daily Monitor newspaper and to the AFP news agency that the massacre had taken place.
"Bodies of the women and children, with deep cuts are littered inside and outside the church," an aid official told The Monitor.
Witness Abel Longi told the AP news agency that he recognised the LRA rebels by their dreadlocked hair, their Acholi language and the number of young boys among them.
"I hid in bush near the church and heard people wailing as they were being cut with machetes," he said.
However, LRA spokesman David Nekorach Matsanga has denied that the rebels are behind the killings.
"Reports about the LRA killing innocent civilians is another propaganda campaign by the Uganda army," he said.
"I have it on good authority from the field commanders that the LRA is not in those areas where the killings are reported to have taken place."
He said the massacre may have been carried out by Ugandan soldiers.
"They want to justify their stay in DRC [Congo] and loot minerals from there like they did before," he told AP.
Capt Magezi said that on Saturday the army had killed 13 of the rebels behind the alleged attack and were pursing the rest of the group.
The UN's humanitarian agency Ocha says 40 people were killed in attacks in DR Congo's Faradje district, 89 around Doruma and 60 in the Gurba area.
Many thousands of Congolese villagers fled their homes after LRA attacks near Dungu in October.
Countries from Uganda to the Central African Republic have suffered 20 years of terror inflicted by the LRA.
Tens of thousands of children have been abducted to be fighters and sex slaves.
Uganda's government said the joint offensive had destroyed some 70% of the LRA camps in DR Congo.
BBC Africa analyst Martin Plaut says that Mr Kony's force is relatively small - about 650 strong - but the difficulty is that when it is hit, it scatters and then regroups.