By Aweys Yusuf

MOGADISHU, Nov 17 (Reuters) - Islamist rebels firing grenades attacked Ugandan peacekeepers early on Saturday in Mogadishu and briefly entered their base, but a spokesman for the African Union force said it suffered no casualties.

The overnight raid triggered a 90-minute battle and came two days after a fugitive Islamist commander believed to be al Qaeda's man in the Somali capital ordered his fighters to target the Ugandans and assassinate their officers.

"We killed one insurgent. His body is still lying there," AU spokesman Captain Paddy Ankunda told Reuters.

"There are no casualties on our side. They attacked us in two groups using machine guns and rocket-propelled grenades and we defended ourselves. We have a mandate to defend ourselves."

About 1,600 Ugandan soldiers have been in Mogadishu since March to support a fragile interim government backed by the West, the United Nations and regional power Ethiopia.

The local authorities and their Ethiopian military allies have faced an Iraq-style insurgency of roadside bombings and political killings since the start of the year when they ousted a hardline sharia courts group from the capital.

Hundreds of thousands have fled fighting this year, and the attacks have also increasingly targeted the peacekeepers -- the vanguard of a long-awaited 8,000-strong AU force.

Showing increasing boldness, some rebels infiltrated the Ugandan base at the strategic K4 junction in the latest attack.

"They shared the buildings we occupy. I don't know how they did that," Ankunda said. "The fighting lasted an hour and a half. But we can't be intimidated and won't withdraw. We're here to assist the Somali people and we shall continue doing so."

Thursday's comments by Islamist commander Aden Hashi Ayro were clearly meant to boost an insurgency that has made Mogadishu one of the world's most dangerous cities.

Ayro, who has been in hiding since the Islamists were routed in January, accused Ethiopia and Uganda of invading and said all other African troops sent to Somalia would face holy war.

Ayro, who security sources say trained in Afghanistan in the late 1990s, also vowed to take his fight to Ethiopia's capital.

"They beheaded our children, women and elderly people in Mogadishu and we must behead theirs in Addis Ababa," he said in the 20-minute recording posted on Somali Web sites. (Writing by Daniel Wallis; Editing by Giles Elgood)

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