A bomb exploded in Somalia's capital Thursday, killing six Somalis, including two police officers, shortly after a group of international journalists was in the area.

United Nations officials said the bomb, believed to be a homemade explosive device, appeared to be targeting a police checkpoint close to a crowded area of makeshift shelters for refugees.

The journalists had just visited a feeding site near the shelters, during a U.N.-sponsored trip to observe international relief efforts. Drought and chronic violence have made Somalia one of the worst humanitarian crises on the planet.

Meanwhile, two Somali insurgents hurled hand grenades at the U.N. compound on Thursday in a separate attack in the restive capital. Officials say the grenades missed the compound.

In response to the worsening violence, the international aid group Doctors Without Borders says it is closing its two largest medical centers in Mogadishu. Two of its workers were killed in the capital last month.

The group said it is shutting down the 120-bed facilities in Mogadishu's Hodan district, where it has been treating malnutrition, measles and cholera.

The director of Doctors Without Borders, Christopher Stokes, said it is hard to stop those services in an area where the group is saving lives, but that the “brutal assassination” of its workers makes it impossible to continue.

Mogadishu is one of the world's most dangerous places for foreign workers.

Somalia has not had a stable government since 1991. The militant Islamist group al-Shabab is attempting to overthrow the U.N.-backed government and impose strict Sharia law over the country.