US troops have begun a region-wide hunt for fighters from the Lord's Resistance Army, a Ugandan-born group that has been killing, raping and looting for years, the Ugandan army said Tuesday.

US President Barack Obama in October sent 100 special forces soldiers to help Uganda track down LRA chief and international fugitive Joseph Kony, who has wreaked havoc over four nations for more than two decades.

"They (US troops) are there and they are setting up their bases," said Ugandan army spokesman Felix Kulayigye.

US troops had deployed to Obo in the Central African Republic and Nzara in South Sudan, where Uganda's army has forward bases to battle the rebel group, Kulayigye said, but gave no details of the numbers of troops sent.

Some of the US troops staged a training exercise Tuesday with Ugandan airforce crews in Entebbe, about 35 kilometres (21 miles) west of the capital Kampala, on how to package supplies to be air dropped to frontline troops.

Previously Uganda had to rely on supplies being ferried in by helicopter to specified landing sites but will now be able to be resupplied without having to return to base, Kulayigye said.

A US official, speaking to AFP here on condition of anonymity because he was not authorised to talk to the press, confirmed that some troops had arrived in affected areas but could not say where exactly the troops were located.

The rebels currently number several hundred, a fraction of their strength at their peak but still include a core of hardened fighters infamous for mutilating civilians and abducting children for soldiers and sex-slaves.

The majority of US troops will be based in Uganda while a smaller number will be based in jungle areas in neighbouring countries to advise regional armies tracking the rebels, US officials say.

The US state department currently gives $17 million each year to cover the cost of transporting Ugandan forces to the conflict zone.

Tens of thousands of people have been killed since Kony took up arms in the late 1980s, initially against the Ugandan government.

The International Criminal Court has a warrant against Kony, one of the continent's most wanted men.

Driven out of Uganda, the guerrillas have since scattered across a vast region of the Central African Republic, Democratic Republic of Congo and South Sudan, recruiting fighters from those nations over the years.

The LRA emerged from the frustrations of Uganda's marginalised Acholi ethnic group against the government, but its leaders have since dropped their national political agenda for the narrow objective of pillage and plunder.