(CNN) -- The Ugandan rebel outfit, the Lord's Resistance Army, abducted almost 700 people in the last 18 months in a largely unreported campaign in the Central African Republic and northern Democratic Republic of Congo, according to a new report by Human Rights Watch.
Almost a third of those who were abducted are children, many of whom were forcibly recruited as soldiers or used for sex by the rebels, the group said.
Human Rights Watch said it interviewed 520 civilians, including 90 who were abducted, in compiling its report.
The rebels killed at least 225 children and adults when they tried to escape, walked too slowly or could not carry heavy loads, the group said.
In some cases, the victims' skulls were crushed with clubs; in others, captive children were forced to kill other children, the group said.
Human Rights Watch said the abuses went largely unreported because of lack of communications in the remote area.
The Lord's Resistance Army was initially based in Uganda, but now works near the borders of Central African Republic, Sudan and Congo.
It has been accused of killing, abducting and mutilating scores of people in the past.
In December, the Lord's Resistance Army carried out a campaign in northeastern Congo over four days, killing at least 321, Human Rights Watch said.
The group is led by self-declared prophet Joseph Kony.
It claims its insurgency aims to replace the Ugandan government with a democracy based on the Ten Commandments. President Yoweri Museveni pushed it out of Uganda in 2005.
Five years ago, the International Criminal Court issued arrest warrants for senior rebel leaders for crimes in northern Uganda, but those indicted remain at large.