(Reuters) - The United States said Thursday it was making progress with its African allies in its push against Uganda's notorious Lord's Resistance Army, but gave no clear end date for the shadowy U.S. military operation unfolding in Central Africa.

President Barack Obama announced in October that he was sending about 100 U.S. troops to help and advise Central African countries battling LRA rebels accused of murder, rape and kidnap in a reign of terror dating back to the late 1980s.

The U.S. troops, mostly Special Forces, hope to speed the hunt for fugitive LRA leader Joseph Kony, and operate with the militaries of Uganda, the Central African Republic, South Sudan and the Democratic Republic of Congo across some of Africa's most remote and hostile terrain.

The LRA, which says it is a religious group, emerged in northern Uganda in the 1990s and is believed to have killed, kidnapped and mutilated tens of thousands of people. Pushed out of Uganda in 2005, LRA fighters now roam remote jungle regions in neighboring states.

Kony has been indicted by the Hague-based International Criminal Court on charges of war crimes and crimes against humanity.

Karl Wycoff, U.S. deputy assistant secretary of state for Africa, said the intervention was showing some positive results, noting that LRA attacks were sharply down and defections were rising.

With our support, these four military forces continue to make some progress in reducing the LRA's numbers and keeping them from regrouping, Wycoff told a briefing at the Center for Strategic and International Studies in Washington.

We believe it is critical that the militaries in the region continue to work together to keep the pressure on the LRA, Wycoff said.

In the last several months, scores of people have defected, escaped or been released from the LRA's ranks. This is a welcome development, Wycoff said, adding that LRA attacks and abductions had also decreased significantly in the latter half of 2011.

ESCALATING PRESSURE

Attempts to negotiate peace failed in 2008 after Kony refused to sign a deal, and past efforts to defeat them militarily have tended to result in brutal retaliation taken out against local villages.

Obama's decision to send U.S. forces to join the fight marked a significant escalation of pressure on LRA, known for hacking body parts off victims and the abduction of young boys to fight and young girls for use as sex slaves.

It also thrust the United States into an expanded role in conflicts in sub-Saharan Africa, putting U.S. troops in the field to support local forces in direct combat with insurgents.

While the U.S. mission has drawn wide support in Congress, which passed a law requiring the United States to do more to fight the LRA, some analysts are skeptical that the new U.S. push will succeed where others have failed.

They have also voiced concern that the United States, by joining up with Ugandan President Yoweri Museveni, is strengthening the hand of an African leader increasingly accused of human rights abuses and political oppression at home.

This is a regime that is basically slowly collapsing from within, said Joel Barkan, a Uganda expert and senior associate in the Africa program at CSIS.

In terms of a long-term bet in respect to providing peace and security in Uganda, a country that held out such promise for a while, those days are over.

'NOT OPEN ENDED'

U.S. officials have underscored the limited nature of U.S. military involvement, and originally said the Africa deployment would last only months and would provide help on tracking, intelligence assessment and conducting patrols

Wycoff said on Thursday that the mission was not open ended but declined to give any further specifics.

The U.S. advisers, armed for self defense but not tasked with a combat role, work primarily with Uganda's military, which has fielded about 1,200 troops in the LRA fight.

The LRA itself is believed to have just a few hundred fighters, but their exceptional mobility and the difficulty of the terrain has made them hard to capture.

Wycoff said the overall military cooperation was going well, with U.S. advisers helping to promote information sharing, training and joint operations planning in a region riven by military and political rivalries.

A U.S.-trained Congolese light infantry battalion working with the U.N. Organization Stabilization Mission in the Democratic Republic of the Congo successfully prevented repeats of Christmas massacres that occurred in 2008 and 2009, he said.

The United States has also backed civilian efforts, such as a high-frequency radio network and new cellphone towers in northern Congo to help local communities trade information and establish a similar community radio correspondence network in the Central African Republic.

Ledio Cakaj, an independent researcher who specializes in the LRA, said the U.S. mission would be under increasing pressure to show results - and that capturing or killing Kony, even if possible, might not resolve the crisis.

There isn't much time left. The rainy season starts in May and July and August are the wettest months. It's very hard to move in the bush, he said, noting that the LRA had been repeatedly underestimated in the past.

You cannot kill them all, and (even if Kony dies) you will still have three or four groups operating. As long as they survive, they succeed.

(Editing by Sandra Maler)

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