Peace talks between Uganda and rebels appeared to stall last month when LRA leader Joseph Kony, wanted for war crimes, failed to appear at a signing ceremony on the Sudan/Congo border.
"Kony and the LRA took advantage of the breathing room given to them and appear to be terrorizing civilians yet again," said Richard Dicker, International Justice Director at Human Rights Watch (HRW).
"Concerned governments and U.N. officials cannot sit by while the LRA goes on a criminal rampage, committing heinous abuses against children and other people," he said in a statement.
Rebel officials were not immediately available for comment.
Kony's 21-year insurgency has claimed tens of thousands of lives and displaced 2 million. Kony and two deputies are wanted for abducting children, massacres and mutilations.
The U.S.-based HRW accused rebels of kidnapping children in mineral-rich eastern Democratic Republic of Congo and oil producing south Sudan as well as Central African Republic.
"Boys are made to act as porters or subjected to military training and girls are being used as sex slaves," the statement said.
"Alleged LRA crimes and Kony's month-long silence intensify questions as to whether the LRA is committed to the negotiations," it added.
Uganda also questioned Kony's commitment to the peace process after he failed to meet a delegation of northern Ugandan elders and Riek Machar, chief mediator and south Sudan vice president.
"Kony made everybody wait for him ... that is not a sign of peace," Uganda's negotiating team spokesman Captain Chris Magezi told Reuters.
"The Uganda Government is waiting for communication from the mediator and the way forward will be based on that."
Kony has been seeking more details on Ugandan government plans to use traditional justice to help him avoid arrest by the International Criminal Court (ICC).