WASHINGTON — President Obama said Friday he has dispatched 100 U.S. troops to central Africa to support a years-long fight against a guerrilla group accused of horrific atrocities.

Obama said they were sent to advise, not engage in combat, unless forced to defend themselves.

In a letter to Congress, Obama said the troops will act as advisers in a long-running battle against the Lord’s Resistance Army, considered one of Africa’s most ruthless rebel groups, and help to hunt down its notorious leader, Joseph Kony.

The first of the troops arrived in Uganda on Wednesday, the White House said, and others will be sent to South Sudan, the Central African Republic and the Democratic Republic of the Congo.

While the size of the U.S. footprint is small, Obama’s announcement represents a highly unusual intervention for the United States. Although some American troops are based in Djibouti and small groups of soldiers have been deployed to Somalia, the United States traditionally has been reluctant to commit forces to help African nations put down insurgencies.

It demonstrates the Obama administration’s escalating attention to and fears about security risks in Africa, including terror networks, piracy and unstable nations. The move was intended to show engagement to lessen the impact of one of the worst protracted wars in Africa.

Obama declared his decision to send troops as in keeping with the national security interests of the United States. The White House announced it in a low-key fashion, releasing the Obama notification and justification of the troop deployment that the president sent to congressional leaders.

Pentagon officials said the bulk of the deployment will be of special operations troops, who will provide security and combat training to African units. The move raises the profile of U.S. involvement on the continent – and represents an apparent victory for administration officials who have argued for more robust intervention in humanitarian crises.

The change in policy could reflect the long-standing concerns of a number of high-ranking Obama advisers left scarred by the U.S. failure in the 1990s to intervene to stop the genocide in Rwanda and the belated action to finally halt the violence in Bosnia. For a current parallel, the Lord’s Resistance Army’s 24-year campaign of rebellion, rape and murder represents one of the world’s worst human rights crises today.

“This case is somewhat exceptional,” said Richard Downie, an Africa expert at the Center for Strategic and International Studies in Washington. “There are few more clear-cut cases of evil in the world today than the Lord’s Resistance Army.”

Downie noted that the United States has once before, at the end of 2008, sent advisers and logistical backup to help the Ugandan army root out the LRA. Intelligence leaks, poor cooperation between the Ugandan and other African armies, and bad weather hampered the operation.

Coming off the administration’s successful, if limited, intervention in Libya, the Uganda deployment represents a continued effort by Obama to use military force for humanitarian protection in areas where atrocities are occurring. Sending 100 troops may not be significant in terms of military numbers, but the composition of the force gives the United States a new counterterrorism foothold in a region of the world with terrorist networks, pirates and unstable nations.