* Al Shabaab says stronger after merging with rival group
* Uganda says ready to counter threat
* Burundi to beef up security over Christmas
(Adds al Shabaab, Uganda, Burundi, AU quotes)
By Abdi Sheikh and Ibrahim Mohamed
MOGADISHU, Dec 23 (Reuters) - Somalia’s al Qaeda-linked Islamist al Shabaab rebels said on Thursday they would increase attacks on Uganda and Burundi after tightening their grip on much of Somalia by joining forces with a rival militant group.
The rebels, who have waged a three-year insurgency aiming to impose a strict form of Islamic law, merged this week with Hizbul Islam, a smaller group which had battled them for control of southern towns.
Between them, they control most of central and south Somalia and much of the capital, hemming Western-backed President Sheikh Sharif Ahmed’s government into a few blocks.
“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’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.”
Uganda and Burundi have troops in Somalia as part of an 8,000-strong African Union (AU) force protecting the government. 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.
In a statement, Al Shabaab said: “We tell our enemies that our swords are out, we are prepared to liberate the global blessed areas from non-Muslims ... our victory is imminent.”
Burundi said it was increasing security around the holidays.
“We take al Shabaab threat very seriously. Security will be tightened during Christmas. Patrols in such places as parks and nightclubs will be multiplied,” Burundi’s minister for public security, Alain Gilaume Bunyoni, told Reuters by phone.
Ugandan police spokesman Vincent Ssekate said the country was prepared to counter the threat: “These kinds of threats are now a kind of song from al Shabaab. We know their strategies and we have good intelligence on their plans.”
The AU force also said it was not cowed by threats.
“We are nearly four years in Mogadishu. How much have Islamists gained from the threats and ultimatum they have given since we arrived?” Barigye Ba-Hoku, AU spokesman told Reuters.
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.
Kenya has also long cast a wary eye at its lawless neighbour, where it fears the growing presence of al Qaeda-linked militants. In 1998, al Qaeda car bombs hit the U.S. embassies in Tanzania and Kenya, killing 224 people.
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 turned on each other. (Additional reporting by Elias Biryabarema in Kampala and Patrick Nduwimana in Bujumbura; Writing by James Macharia; Editing by Peter Graff)