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WASHINGTON -- President Obama is sending about 100 U.S. troops to central Africa to help local forces battle the Lord's Resistance Army, a rebel group that the administration says has waged a campaign of murder, rape and kidnapping for more than two decades.

Obama said Friday the troops will act as advisers in efforts to hunt down rebel leader Joseph Kony but will not engage in combat except in self-defense, according to a letter to Congress that was obtained by Fox News.

The White House says the first troops arrived in Uganda on Wednesday. Ultimately, they will also deploy in South Sudan, the Central African Republic and Congo.

A senior administration official downplayed the notion that the armed troops could be drawn into a hostile, combat situation, saying the move was sparked by Congress passing a law year urging the administration to do something to crack down on the Lord's Resistance Army.

Long considered one of Africa's most brutal rebel groups, the Lord's Resistance Army began its attacks in Uganda more than 20 years ago but has been pushing westward.

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    The administration and human rights groups say its atrocities have left thousands dead and have put as many as 300,000 Africans to flight. They have charged the group with seizing children to bolster its ranks of soldiers and sometimes forcing them to become sex slaves.

    Kony is wanted by the International Criminal Court under a 2005 warrant for crimes against humanity in his native Uganda.

    Obama's announcement came in low-key fashion -- a letter to the leader of the House, Speaker John Boehner, in which he said the deployment "furthers U.S. national security interests and foreign policy and will be a significant contribution toward counter-LRA efforts in central Africa."

    The deployment drew support from Sen. James Inhofe, a Republican who has visited the region.

    "I have witnessed firsthand the devastation caused by the LRA, and this will help end Kony's heinous acts that have created a human rights crisis in Africa," he said in a statement. "I have been fervently involved in trying to prevent further abductions and murders of Ugandan children, and today's action offers hope that the end of the LRA is in sight."

    But Obama's letter stressed the limited nature of the deployment.

    "Our forces will provide information, advice and assistance to select partner nation forces," it said. "Although the U.S. forces are combat-equipped, they will ... not themselves engage LRA forces unless necessary for self-defense."

    The troops deployed to central Africa will be mostly U.S. special operations forces. It's likely many of these forces will be Army Green Berets, one military official told Fox News, but it's unlikely that will be announced publicly.

    Since 2008, the U.S. has provided $33 million to regional forces battling LRA, according to the Pentagon.

    The Pentagon said the special operation forces are performing one of their core missions.

    "Our intention is to provide the right balance of strategic and tactical experience to supplement host nation military efforts," the Pentagon said. "Ultimately, Africans are responsible for African security, but we remain committed to our partners to enable their efforts to provide for their own security."

    Fox News' Ed Henry, Justin Fishel and The Associated Press contributed to this report.