(Adds claim for bombing that killed a Burundian soldier)

By Abdi Shekih and Aweys Yusuf

MOGADISHU, April 9 (Reuters) - Islamist fighters in Somalia seized a strategic town north of Mogadishu on Wednesday for the second time in two weeks, a spokesman for the insurgents said.

Jowhar is the most significant of several towns the rebels have captured in recent months, highlighting the inability of the Western-backed interim government to impose its authority despite support from Ethiopian and African Union (AU) troops.

The Islamists, who are remnants of a sharia courts group ousted from the capital at the end of 2006, briefly seized Jowhar on March 26. Then early on Wednesday, they did it again.

"No fighting took place because the enemy troops had abandoned the town by midnight when they heard we were coming," the Islamists' spokesman Abdirahim Isa Adow said by telephone.

The rebels freed prisoners in Jowhar, which served as the government's temporary base in 2005, Adow told Reuters.

Local Somali broadcaster Shabelle said the Islamist fighters wore turbans and chanted "Allahu akbar" (God is greatest). Residents said they later left the town.

In recent months the insurgents have seized towns from local administrations that often amount to little more than militias, only to give them up and melt away -- or be routed by Ethiopian or Somali government forces who arrive later.

As the rebels step up their attacks, a spokesman for Burundian troops serving with the AU peacekeeping force in Mogadishu said one of their soldiers was killed on Tuesday by a suicide bomber who rammed a car into the gates of an AU base.

Witness Fahom Mohammed said four civilians also died.

"A refugee woman and her three children were cut to pieces by the blast while they were queuing outside the Burundian compound waiting for water," she told Reuters.

The Young Mujahideen Movement, an Islamist Somali group, claimed responsibility for the bombing. In a statement posted on a Web site used by Islamists, the group also threatened more attacks against African Union forces.

Elsewhere in Somalia's rubble-strewn capital, at least two Ethiopian troops were wounded on Wednesday when a remote controlled roadside bomb tore through their convoy.

"I saw two Ethiopian soldiers bleeding heavily and lying in the road," said local man Ahmed Shine. "The Ethiopians blocked off the whole area where the incident happened." (Additional reporting by Sami Aboudi in London; Writing by Daniel Wallis) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/)

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