Mortar shells likely fired by insurgents crashed into civilian homes overnight in Somalia's capital, killing seven people, including four children, witnesses said Tuesday.

A father, mother and their two children died in one home, and a mother and her 8-year-old child died in a nearby house, said Abdinur Hashi, a resident in the Hodan district of Mogadishu. A fourth child also died after suffering serious wounds caused by shrapnel.

Some of the bodies were wrapped in white shrouds at daylight. Hashi called the violence a "catastrophe."

"Sleeping, innocent children and their parents were killed here," Hashi said as he stood over the bodies.

Security in Mogadishu has improved markedly over the past year, after African Union and Somali troops pushed al-Shabab insurgents out. But indiscriminate mortar and bomb attacks still take place.

"It was a shocking loss. Why are such poor people being attacked?" Ubax Abdi, a neighborhood resident, said as she wept.

A similar attack in March killed six people in what appeared to be a mortar assault aimed at the presidential palace.

Al-Shabab, Somalia's most powerful insurgent group, routinely denies that its mortars kill civilians while claiming that the attacks hit their intended target, such as the presidential palace.

Somalia has been mired in conflict since the 1991, when long-term dictator Siad Barre was overthrown by warlords who then turned on each other. Al-Shabab has had a grip on much of south-central Somalia for the last several years.