Somali Web sites that posted the 20-minute recording named the speaker as Aden Hashi Ayro, who has been in hiding since his sharia courts group was ousted from its southern strongholds by joint Somali-Ethiopian forces in January.

One of a group targeted by a U.S. air strike that month, the militant urged Somalis to wage holy war on foreign troops and accused Uganda of invading by sending 1,700 peacekeepers to Somalia -- the vanguard of a planned 8,000-strong AU force.

"Like Ethiopia, Uganda also invaded our country. We will fight and assassinate their officers. All other African troops sent to Somalia will face the same fate," he said on the tape, the authenticity of which could not be independently verified.

"We promise as long as we are alive we will continue the jihad."

Uganda said it was not deterred by Ayro's threats, which surfaced as Burundi prepared to deploy hundreds of troops to the Horn of Africa nation where anarchy has ruled for 16 years.

"It does not change anything," said Uganda's army spokesman, Major Felix Kulayigye. "If anything, we expected to receive threats against us."

"BEHEADED OUR CHILDREN"

Ayro, who security and intelligence sources say trained in assassination and insurgency tactics in Afghanistan in the late 1990s, vowed to take the fighting to the Ethiopian capital.

"They beheaded our children, women and elderly people in Mogadishu and we must behead theirs in Addis Ababa," he said.

Security experts and diplomats say Ayro, in his 30s, has provided shelter and weapons to al Qaeda members in Mogadishu.

He prefaced his message with a greeting to all Mujahadeen groups and al Qaeda leader Osama bin Laden.

Somalia's interim government accuses Ayro's al-Shabab militia of leading the insurgency in Mogadishu, where battles this year have killed thousands and uprooted hundreds of thousands more, triggering what a U.N. envoy called Africa's worst humanitarian crisis.

Staging roadside bombings and political killings, the rebels are increasingly borrowing tactics from jihadists in Iraq.

"My only word to al-Shabab ... is they should lay down their arms. That's what the majority of Somalis want," Kulayigye said.

Despite securing the airport, seaport and presidential palace, along with a few roads and clinics, the Ugandans have otherwise been unable to do much to quell the violence.

Analysts say Kampala's mandate to defend the country's Ethiopian-backed Transitional Federal Government means it is seen as partisan by many Somalis.

"We can't be partisan. We have not been involved in the fighting," Kulayigye said. "We've done what's within our mandate and there's nothing more we can do."