MOGADISHU (Reuters) - Somalia's hardline Islamist insurgent group al Shabaab pledged Monday to launch more attacks on African Union peacekeepers after the deadliest strike yet killed at least 11 soldiers from Burundi.

This is our land and you are non-believers, said a statement on a website used by the militants, who are fighting the Somali government and a 3,500-strong AU peacekeeping force.

Leave us for your safety or we shall never tire of increasing your death toll.

The site,, posted pictures of two young men it said were suicide bombers who blew up explosives in a jacket and a car next to an AU base at a former university in the coastal capital Mogadishu.

The militants' Internet statement said 52 people died and 34 were wounded in Sunday's attack.

The AU said its compound had been targeted by mortar bombs, not suicide bombers. It said 11 were killed and 15 injured.

Witnesses described a car speeding toward the gate before hearing at least one blast and seeing thick plumes of smoke.

Somalia's new President Sheikh Sharif Ahmed, a moderate Islamist, arrived in the rubble-strewn city on Monday and condemned the latest bloodshed.

AU troops will not leave through fighting and explosions, but through peaceful negotiation among the Somalis, he said.

His new prime minister, Omar Abdirashid Ali Sharmarke, the Western-educated son of a slain former president, was also due in Mogadishu later Monday.

Both men have been in neighboring Djibouti picking a cabinet under U.N.-brokered efforts to form a unity government to end 18 years of conflict in the failed Horn of Africa state.


Their biggest threat is from al Shabaab which, together with allied militia, controls large swathes of southern Somalia including the strategic towns of Baidoa and Kismayu.

The government controls only parts of Mogadishu.

Since the start of an Islamist insurgency that broke out at the beginning of 2007, at least 16,000 civilians have been killed and a million more driven from their homes.

Al Shabaab gained support as one of many groups waging war against Ethiopian troops propping up the previous government.

An Ethiopian withdrawal in January placated some Somalis, but al Shabaab has now turned its fire on the AU peacekeeping mission, AMISOM, and the new government.

The government of Burundi, which contributes nearly half AMISOM's strength, said it remained committed to stabilizing Somalia and planned to reinforce its contingent soon. Burundi's military said the mission's current mandate was too restrictive.

In neighboring Kenya, the AU special envoy to Somalia, Nicolas Bwakira, said AMISOM's current mandate was sufficient. Somali troops would be used for peace-enforcement, he said.

Two more battalions, one from Uganda and another from Burundi, are due to deploy within weeks, bringing the force to more than 5,000 -- but far short of its planned 8,000.

Bwakira said Malawi, Nigeria and Ghana had all agreed to send troops. But most African nations have been reluctant to send their soldiers into harm's way in Somalia.

Experts hope the inclusion of moderate Islamists in the new Somali government may marginalize hardliners like al Shabaab. The group is on Washington's list of terrorist organizations and is known to have foreign fighters in its ranks.

But analysts are split over al Shabaab's strength.

Some say it could overrun the government, while others say it has only a few thousand fighters and has used the media and high profile strikes to project a more powerful image -- despite waning support among traditionally moderate Somali Muslims.

In a recording From Kabul to Mogadishu posted online on Monday, al Qaeda's second-in-command Ayman al-Zawahri urged Somalis not to fall for a secular constitution and said militants would fight the U.S.-made government. [nLN525094]

Somali gunmen freed a Pakistani man Monday a day after kidnapping him in the semi-autonomous northern region of Puntland, officials said.

Abdullahi Said Samatar, Puntland's security minister, told Reuters no ransom had been paid and that the man was released after local elders made contact with the gunmen.

(Additional reporting by Abdi Guled in Mogadishu, Abdiaziz Hassan and Alison Bevege in Nairobi and Patrick Nduwimana in Bujumbura; Writing by Andrew Cawthorne and Daniel Wallis)