US special forces will be deployed with front-line units as advisors to central African militaries hunting down the leaders of Uganda's Lord's Resistance Army, a senior US official said Tuesday.

Alexander Vershbow, a top Pentagon official, assured lawmakers that the deployment would likely last no more than "months" and was limited to aiding regional militaries to gather and use intelligence more effectively to target the elusive rebel force.

But he said some of the estimated 100 US troops would be advising regional militaries in the field -- possibly in small combat units -- and would be armed and authorized to use their weapons in self-defense.

"The bulk of the overall roughly 100 people would be in Uganda, but small teams would deploy forward in partnership with the local forces to sort of help them improve their skills on the front line," he said.

Vershbow said the US troops would be deployed wherever they were thought to be most effective, such as in brigade headquarters but also possibly with platoons, the small infantry units that are most likely to see combat.

President Barack Obama notified Congress October 14 that he was sending the troops to train and advise African forces with the goal of capturing or removing the leaders of the LRA, a Ugandan rebel group notorious for using children as fighters or as sex slaves and porters.

LRA rebels are accused of terrorizing, murdering, raping and kidnapping thousands of people, and tens of thousands of people have died in their 20-year war with security forces in northern Uganda.

A key target of the US operation is Joseph Kony, the group's messianic leader who is believed to be ensconced in the jungles of the Central African Republic with a force that US officials say numbers only around 200 core fighters and a total 800 followers.

They have dispersed from northern Uganda in small, hard to locate units in the Central African Republic, the Democratic Republic of the Congo and South Sudan, where they have carried out a series of deadly attacks over the past two years.

While a 2010 law requiring a US strategy to dismantle the group gained broad bipartisan support in the US Congress, some lawmakers expressed concern at a hearing Tuesday about the potential for mission creep in an open-ended deployment.

"I have a lot of anxious moments about whether or not the number of troops won't grow to 200-300 or even more," said Representative Donald Manzullo, a Republican from Illinois.

Others, Republicans as well as Democrats, voiced strong support for the mission.

"Sometimes just getting rid of one person does make a big difference. History is full of captivating leaders with bad ideas who do great damage," said Representative Ed Royce, a Republican from California.

"Kony's removal won't guarantee peace, but it is the one thing that makes peace possible in that region," he said.

Vershbow, the assistant secretary of defense for International Security Affairs, and Donald Yamamoto, the principal deputy assistant secretary of state for African Affairs, were peppered with questions about the cost of the deployment, and how long it would last, but were unable to say.

"We don't have a specific timetable. We're talking months, but I wouldn't put a number on it at this point. But we will be reviewing the operation in a few months to see whether it is achieving the desired effect," Vershbow said.

He defined success as "first and foremost... whether Kony and other commanders are actually captured, whether we see further fracturing of the LRA and more defections, whether we see tangible improvement in our partners' capacities out in the field to succeed."

Vershbow said a focus of the US mission would be to improve intelligence gathering and intelligence sharing among the militaries from the four African countries, whose past failures to coordinate have stymied previous operations against the LRA.

Yamamoto said the United States was working to get the presidents of Uganda and the DRC to coordinate their forces against the LRA.

The operation does not envisage the use of drones for intelligence gathering, Vershbow said. But he declined to comment in open session on whether they might be used for air strikes.

"I think that the kind of intelligence that is most important to the success of this operation is the human intelligence gathered on the ground," he said. "That depends on closer ties between the military forces of the countries involved and the local population."