Mogadishu - At least eight Somali government troops were killed when a remote-controlled bomb hit their vehicle in Mogadishu, officials said on Thursday, highlighting lingering security risks less than two weeks before the U.N.-backed government's mandate ends.

Somalia's Islamist al Shabaab group claimed responsibility for the attack on Wednesday evening.

Government troops and African Union peacekeepers say they have tightened security before a presidential election on August 20 when the transitional government will be dissolved.

A combined force including Kenyan, Burundian, Ethiopian and Djiboutian troops is planning an offensive on Kismayu, Somalia's second biggest city and a hub for al Shabaab, before then.

Residents said the military vehicle had been destroyed by a roadside bomb in the Huriwaa district of northern Mogadishu.

"A bomb killed eight soldiers and completely destroyed their pickup. Only two, including the driver, survived with injuries," Mohamed Abdikadir, a senior police officer, told Reuters.

Although Somali troops and peacekeepers of the African Union Mission in Somalia (AMISOM) drove al Shabaab fighters from most of Mogadishu in August 2011, the latest bombing shows the militants can still stage deadly attacks in the capital.

Al Shabaab has threatened more attacks as Somalia's three top government officials and a dozen other politicians campaign for the presidency. Last week the government foiled two would-be suicide bombers who targeted a conference hall where delegates approved a draft constitution.

"We destroyed the government pickup yesterday," Sheikh Abdiasis Abu Musab, military spokesman for al Shabaab, told Reuters. "AMISOM and government forces will never sleep or relax. More explosions await them."

On Tuesday, Uganda's military, which provides the bulk of peacekeeping force, said it had deployed an air force contingent to reinforce the troops in their fight against al Shabaab.