ABC News’ Jake Tapper and Luis Martinez report:

Two days ago President Obama authorized the deployment to Uganda of approximately 100 combat-equipped U.S. forces to help regional forces “remove from the battlefield” – meaning capture or kill – Lord’s Resistance Army leader Joseph Kony and senior leaders of the LRA.

The forces will deploy beginning with a small group and grow over the next  month to 100. They will ultimately go to Uganda, South Sudan, the Central African Republic, and the Democratic Republic of the Congo, with the permission of those countries.

The president made this announcement in a letter to House Speaker John Boehner, R-Ohio, Friday afternoon, saying that “deploying these U.S. Armed Forces furthers U.S. national security interests and foreign policy and will be a significant contribution toward counter-LRA efforts in central Africa.”

He said that “although the U.S. forces are combat-equipped, they will only be providing information, advice, and assistance to partner nation forces, and they will not themselves engage LRA forces unless necessary for self-defense.”

The president said that for more than two decades the LRA has been responsible for having “murdered, raped, and kidnapped tens of thousands of men, women, and children in central Africa” and continues to “commit atrocities across the Central African Republic, the Democratic Republic of the Congo, and South Sudan that have a disproportionate impact on regional security.”

A senior Defense official says the 100 military personnel will be mostly Special Operations Forces and that they “will be traveling  out to field locations in the areas affected by the LRA where they can interact with and advise those forces that are actively pursuing the LRA.”   The official stressed, “they will not be engaging in direct combat against the LRA.”

The US has been helping the four African nations counter the LRA for several years by providing local militaries with training and equipment.  In the Democratic Republic of the Congo the US helped train a light infantry battalion deployed to fight the LAR and  over the last three years in Uganda the US has provided $33 million to help Uganda’s military.

As for how long the US troops will be in the region, a spokesman at US Africa Command says he could not provide specifics, “but our forces are prepared to stay as long as necessary to enable regional security forces to carry on independently.

The president in his letter noted that Congress passed “the Lord’s Resistance Army Disarmament and Northern Uganda Recovery Act,” signed into law on May 24, 2010, in which, the president said, “the Congress also expressed support for increased, comprehensive U.S. efforts to help mitigate and eliminate the threat posed by the LRA to civilians and regional stability.”

When the president signed that letter in May 2010,  he said the bill “crystallizes the commitment of the United States to help bring an end to the brutality and destruction that have been a hallmark of the LRA across several countries for two decades, and to pursue a future of greater security and hope for the people of central Africa. The Lord’s Resistance Army preys on civilians – killing, raping, and mutilating the people of central Africa; stealing and brutalizing their children; and displacing hundreds of thousands of people.  Its leadership, indicted by the International Criminal Court for crimes against humanity, has no agenda and no purpose other than its own survival.  It fills its ranks of fighters with the young boys and girls it abducts.  By any measure, its actions are an affront to human dignity.”

The act passed both houses of Congress with overwhelming support on May 10, 2010 with language that included “providing political, economic, military, and intelligence support for viable multilateral efforts to protect civilians from the Lord’s Resistance Army.”

Human Rights Watch has a great deal of information about the infamous LRA.

Update 4:30 p.m.: This piece has been updated with new information