Somali Islamist gunmen ambushed a convoy of government officials and journalists Friday during intense fighting in Mogadishu, killing three soldiers and an official, an Agence France Presse photographer said.

"We were heading to the frontline when our convoy was ambushed by Shebab fighters, an RPG (rocket-propelled grenade) struck one vehicle," the photographer said, adding that he saw four bodies.

African Union-backed Somali government forces launched a fresh offensive Friday to seize holdout Islamist Shebab positions in the war-torn capital Mogadishu, officials and witnesses said.

Heavy fighting broke out as troops supported by tanks and artillery from the African Union Mission in Somalia (AMISOM) moved into the capital's northern Heliwa and Gubta suburbs, still held by the al-Qaida linked Shebab.

Aid group Doctors Without Borders (Medecins Sans Frontieres - MSF) said Thursday it had halved its operations in Somalia's war-torn capital after two employees were shot dead last month.

MSF also said it closed down operations in Mogadishu's Hodan district, where it was helping some 200,000 people who fled other Somali regions hit by drought and famine.

Unidentified men attacked the main United Nations compound in the Somali capital Mogadishu, hurling two hand grenades that exploded near the wall, U.N. officials and witnesses said Wednesday.

According to witnesses, two grenades struck by the roadside next to the wall of the U.N. compound late Tuesday. Security guards in the compound opened fire in response, but no casualties were reported.

U.N. chief Ban Ki-moon urged Somalia's al-Qaida-linked insurgents to end violence during a surprise visit Friday to war-torn Mogadishu, where he arrived wearing a bullet-proof jacket.

Flanked by a guard wearing a white U.N. helmet, Ban was welcomed at the airport by Prime Minister Abdiweli Mohamed Ali on the first visit by a U.N. chief to Mogadishu, often described as the world's most dangerous city, in almost two decades.

A suicide bomber blew himself up Tuesday in central Mogadishu after being arrested, killing two soldiers and wounding at least four civilians, a security official said.

"A man was detained by soldiers driving a car we believed was full of explosives, but when the soldiers told the driver to open the car, he detonated explosives on his body," said government security official Abdi Hashi.

Three people were wounded in a suspected bomb blast inside the main hospital in the war-torn Somali capital, officials and witnesses said Monday.

The explosion occurred late Sunday inside a room near one of the main meeting halls of Banadir hospital.

Three Somali soldiers and a civilian were killed Friday when a roadside bomb the officers picked up to detonate elsewhere exploded inside their vehicle, officials and witnesses said.

The soldiers picked up the device at a road intersection in Mogadishu's Bakara district and moments later it ripped apart their vehicle, also killing one civilian and injuring four others who were nearby.

A Kenyan soldier was killed and four seriously wounded Thursday when an explosive device blew up their truck close to the border with war-torn Somalia, an army spokesman said.

"Five KDF (Kenya Defense Force) soldiers were seriously injured and have been airlifted to Garissa for treatment," Major Emmanuel Chirchir said, adding that one later "succumbed to his injuries."

An American citizen of Somali origin was one of the two suicide bombers behind a twin attack on a pro-government military base in Mogadishu, the Islamist Shebab movement said on Sunday.

Several pro-Shebab radio stations broadcast what they said were the last words of Abdusalam al-Muhajir who said that he "wanted to die as a martyr" after spending the last two years fighting alongside the rebels.