* Al Shabaab says stronger after merging with rival group
* Extremists claimed July twin bomb strike on Kampala
* Kenya, Uganda, Tanzania issue security alerts
(Adds Hizbul Islam quote, background)
By Abdi Sheikh and Ibrahim Mohamed
MOGADISHU, Dec 23 (Reuters) - Somalia&${esc.hash}39;s al Qaeda-linked al Shabaab rebels said on Thursday they would increase their attacks against Uganda and Burundi after the Islamist rebels joined forces with rival extremists Hizbul Islam.
Al Shabaab claimed responsibility for twin suicide bombings in the Ugandan capital Kampala on July 11, which killed 79 people watching the World Cup final on television.
It was Shabaab&${esc.hash}39;s first attack outside Somalia and heightened concerns about its ability to carry out more attacks in the region and beyond.
The rebels said they would hit out at Uganda and Burundi for having their troops in Mogadishu as part of African Union&${esc.hash}39;s (AU) peace force AMISOM, which is trying to bring stability to the anarchic Horn of Africa country.
"We, al Shabaab and Hizbul Islam have united and we warn Uganda and Burundi forces and their people that we shall redouble our attacks," Sheikh Ali Mohamud Rage, al Shabaab&${esc.hash}39;s spokesman told a media conference in Mogadishu.
"We also inform our brothers in Afghanistan, Iraq, Nigeria and Uganda, that we have united in one name -- al Shabaab."
Kenya, Uganda and Tanzania have issued security alerts after a grenade exploded at a bus depot in Nairobi on Monday.
Uganda said on Monday it had received intelligence reports than an al Qaeda-linked group was planning to hit the east African country during the festive season.
"Starting from today we are al Shabaab, we have joined them," Abdifatah Moalim, an official who represented Hizbul Islam said.
Twice hit by al Qaeda-linked attacks, Kenya has long cast a wary eye at its lawless neighbour Somalia, where al Shabaab militants have been waging a three-year insurgency against the Western-backed Somali government and want to impose a harsh version of Islamic law.
In 1998, al Qaeda car bombs hit the U.S. embassies in Dar es Salaam and Nairobi, killing 224 people.
Al Shabaab and Hizbul Islam control most of central and south Somalia as well as much of the capital, hemming Western-backed President Sheikh Sharif Ahmed&${esc.hash}39;s beleaguered government into just a few blocks in Mogadishu.
The AU force, which backs Ahmed, has said the merger of the rebel groups could lead to more violence.
More than 21,000 civilians have been killed in Somalia since the al Shabaab insurgency started in 2007.
Lawless Somalia has not had a functioning government since 1991, when rival warlords overthrew dictator Mohamed Siad Barre and then turned on each other. (Writing by James Macharia; Editing by Jon Hemming)
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