Assailants believed to be members of armed groups opposed to Somalia's transitional government and its Ethiopian allies have killed four police officers in the capital, Mogadishu, in continued fighting described by human rights organisations as mostly affecting civilians.

"These elements that are opposed to peace in Somalia targeted the police in an attack on Monday afternoon [13 August] and killed four of them and wounded several other people," Abdi Haji Gobdon, spokesman for the Transitional Federal government (TFG) told IRIN.

He said the attackers struck a police car with an explosive device, killing the officers. The "aggressors" used "hit and run" tactics to disrupt the government's efforts to maintain law and order in the city, he added, saying that the blast, which was in a busy street, could have killed or wounded several civilians.

"They are throwing explosives into the city. Definitely people will be killed or wounded," said Gobdon.

International news agencies reported that 30 civilians had been killed in different incidents over 24 hours – including a bomb attack on a public minibus in Mogadishu – and that 60 people were wounded.

Human Rights Watch (HRW) said in a report released on 13 August that the insurgents were putting civilians at great risk by launching attacks against government forces and Ethiopian troops from residential areas.

Two senior journalists were killed in separate attacks on 11 August, prompting widespread condemnation of the murder and calls for the international community to act to protect press freedom in Somalia.

Violence in Mogadishu has driven hundreds of thousands of civilians from the city this year, forcing them to live in squalid camps in the outskirts of the capital, where they have limited access to food and water, and lack shelter, medical and sanitation facilities.