Ceasar Achellam, considered the fourth-highest ranking member of the LRA, was arrested by Ugandan forces.
Uganda says it has captured a top commander of the Lord's Resistance Army, the guerrilla movement notorious for its attacks on civilians and use of child soldiers.
Ugandan commanders displayed Caesar Achellam to reporters after his capture in what they said was a weekend raid in the Central African Republic. The CAR is one of several African Union countries that has committed troops to hunt down LRA chieftain Joseph Kony, who is wanted for war crimes by the International Criminal Court.
"This is a big fish," said Col. Felix Kulayigye, a Ugandan military spokesman. "For Caesar to be in our hands it is a big statement, as far as our efforts to end this rebellion."
Achellam told reporters he hoped his capture would lead "my people remaining in the brush" to give up, "so that sooner maybe the war would come to an end."
Radhika Coomaraswamy, U.N. special representative for Children and Armed Conflict, called on Ugandan authorities Monday to "not apply amnesty but instead, bring him (Achellam) to justice."
"The arrest and subsequent prosecution of Achellam would send a strong message to the LRA leadership that they will be held accountable for their actions," Coomaraswamy said.
Achellam was captured with his wife, child and a 12-year-old girl from the Central African Republic, according to the U.N.
Achellam and his family are in Ugandan custody in South Sudan. The unidentified girl remains in the Central African Republic. It's unknown why she was with Achellam, the U.N. said.
The hunt for Joseph Kony
The 'Kony 2012' phenomenon
Villagers in Uganda watch 'Kony 2012'
Kony led a failed uprising against the government of Uganda and was pushed out of Uganda in 2006. He has been moving around other countries in the region ever since.
Abou Moussa, a special U.N. envoy for central Africa, told CNN in March that Kony may be in the Central African Republic with between 200 and 700 remaining troops.
Kony is accused of using vicious tactics to recruit children to use them as soldiers and sex slaves and of slicing off ears, noses and limbs of his victims. There are reports of child soldiers brainwashed into killing their own parents.
A celebrity-backed video that went viral earlier this year helped make Kony's alleged crimes more widely known. The half-hour documentary "KONY 2012" was viewed more than 89 million times on YouTube, but the video also spurred a flurry of questions about its producers' intentions, their transparency and whether the social-media frenzy was too little, too late