US President Barack Obama has said he is sending about 100 US soldiers to Uganda to help regional forces battle the notorious Lord's Resistance Army.
Although combat-equipped, the troops would be providing information and advice "to partner nation forces", Mr Obama wrote in a letter to US Congress.
A small group is already in Uganda, and the troops could later be deployed in other central African nations.
The LRA is blamed for mass murder, rape and kidnapping in the region.
'Kill or capture'
"I have authorised a small number of combat-equipped US forces to deploy to central Africa to provide assistance to regional forces that are working toward the removal of (LRA leader) Joseph Kony from the battlefield," Mr Obama wrote on Friday.
But he stressed that "although the US forces are combat-equipped... they will not themselves engage LRA forces unless necessary for self-defence".
Mr Obama did not provide any details about the deployment duration, but a US military spokesman later told the BBC that the "forces are prepared to stay as long as necessary to enable regional security forces to carry on independently".
The force will use hi-tech equipment to assist in what analysts say is a "kill or capture" policy, the BBC's Marcus George in Washington reports.
The deployment follows recent US legislation to help disarm the LRA and bring its leader to justice. The theory is, our correspondent adds, that without Joseph Kony, the movement will collapse from within.
Senator John McCain said Central Africa would be more stable if the threat of the LRA "under the sadistic leadership of Joseph Kony," would be "diminished".
But Mr McCain, a long-serving senator, veteran and Mr Obama's opponent in the 2008 presidential election, expressed "regret" that the president did not consult with Congress on the decision to sent troops to Uganda.
"I remember how past military deployments intended to further worthy humanitarian goals, whether it was peace-keeping operations in Lebanon or Somalia, resulted in tragedies that we never intended or expected," Mr McCain said in a statement.
At least 30,000 people died as the LRA spread terror in northern Uganda for more than 20 years, displacing some two million people.
It is notorious for kidnapping children, forcing the boys to become fighters and using girls as sex slaves.
The group is listed by the US as a terrorist organisation and now operates mainly in neighbouring countries such as Democratic Republic of Congo, South Sudan and Central Africa Republic.
Joseph Kony and his close aides have been wanted by the International Criminal Court (ICC) since 2005.
He refused to sign a peace deal with the Ugandan government in 2008 when it could not guarantee the withdrawal of the ICC arrest warrants.