(CNN) -- A rebel group in the Democratic Republic of Congo killed at least 321 civilians and abducted 250 others -- including at least 80 children -- in a previously unreported rampage late last year, Human Rights Watch said in a report released Saturday.
The Lord's Resistance Army (LRA) carried out the brutal campaign in northeastern Congo over four days in December, the report said.
LRA forces attacked at least 10 villages from December 14 to 17, killing and abducting hundreds of civilians -- including women and children, according to Human Rights Watch.
LRA combatants tied up villagers in the nation's remote Makombo area and hacked them to death with machetes or crushed their skulls with axes and heavy wooden sticks, the report said.
Most of those killed were adult men, but at least 13 women and 23 children were among the dead -- including a 3-year-old girl who was burned to death, according to Human Rights Watch.
The LRA also killed those they abducted who walked too slowly or tried to escape, Human Rights Watch said.
According to those who managed to escape, children captured by the LRA were forced to kill other children who had disobeyed the LRA's rules, the report said. In numerous cases, children were ordered to surround the victim in a circle and take turns beating the child on the head with a large wooden stick until the child died, the report said.
"The Makombo massacre is one of the worst ever committed by the LRA in its bloody 23-year history, yet it has gone unreported for months," said Anneke Van Woudenberg, a senior Africa researcher at Human Rights Watch. "The four-day rampage demonstrates that the LRA remains a serious threat to civilians and is not a spent force, as the Ugandan and Congolese governments claim."
CNN could not independently confirm the massacre.
Human Rights Watch said that the roughly 1,000 United Nations peacekeeping troops in LRA-affected parts of northeastern Congo are insufficient to protect civilians. The peacekeeping force is considering removing some troops from the area under pressure from the Congolese government, a move Human Rights Watch warned against on Saturday.
The U.N. Security Council is planning to visit Congo in mid-April to discuss the peacekeeping force's plans for withdrawal and the protection of civilians, Human Rights Watch said.
The Congolese government denies that the LRA is still a serious threat in the country, which may have contributed to the absence of reports about the December massacre, Human Rights Watch said.
"We have been forgotten," an 80-year-old Congolese man whose son was killed during the massacre told Human Rights Watch. "It's as if we don't exist."
"The government says the LRA are no longer a problem, but I know that's not true," he said. "I beg of you, please talk to others about what has happened to us."
The LRA is led by self-declared mystic and prophet Joseph Kony, who claims his insurgency -- which began in 1986 -- is aimed at replacing Uganda's government, led by President Yoweri Museveni, with a democracy based on the Bible's Ten Commandments.
After being pushed out of Uganda in 2005, the LRA now operates in the remote border area between southern Sudan, Congo, and Central African Republic. In 2005, the International Criminal Court issued arrest warrants for senior LRA leaders for crimes they committed in northern Uganda, but those indicted remain at large.
The two commanders who perpetrated the December massacre report to one of those indicted leaders, according to Human Rights Watch.
The Makombo massacre is the deadliest documented attack by the LRA since killing sprees around Christmas 2008 left scores of Congolese dead, but dozens of other attacks against civilians have been carried out in other areas in recent months, Human Rights Watch said.