Somali government forces attacked rebel strongholds in Mogadishu on Wednesday, triggering battles that killed at least 17 people, including the capital's police chief, witnesses and officials said.

Residents cowered in their homes or took cover behind buildings as mortars slammed into the city.

Islamist fighters wearing headscarves and ammunition belts draped over their shoulders were seen arriving from the outskirts of the capital to join the battle.

The government only controls a few blocks of Mogadishu with the help of an African Union peacekeeping force that guards the air and sea ports and other key government installations. Different Islamic insurgent groups control the rest of Mogadishu, and their goal is to topple Somalia's Western-backed government and install a strict Islamic state.

"A mortar landed on a neighbor's house and killed two people and injured four others," witness Abdiwali Dahir said from one of the two areas of southern Mogadishu where the fighting was under way.

Police spokesman Col. Abdullahi Hassan Barise said Mogadishu police chief Col. Ali Said died in the fighting but was unable to provide details about how that had happened. Said had led the capital's police force for about two years.

Another witness, Farah Abdi, said he saw five corpses lying in the capital's streets. Speaking in a telephone interview as the sound of heavy gunfire echoed in the background, Abdi said three of the victims were civilians and that the two others appeared to be an Islamist fighter and a government soldier.

An administrator at Mogadishu's Medina Hospital, Ali Ade, said it had received 39 wounded people, with three of them dying.

In another part of southern Mogadishu, an Associated Press reporter saw six bodies lying in a street outside a home that had been hit by a mortar. A grieving Somali woman identified one of the victims as her son.

"A stray mortar landed on these young men who sought refuge in our house," said the woman, Hawo Hassan. "It is a tragic day. He was my eldest son, my loved son, my dear son," Hassan said as she cried and held her head.

Mogadishu resident Hanad Abdi Garun, who was huddled in his house, said government forces brought in reinforcements overnight before they started the offensive against Islamist bases.

Another resident, Asha Moalim, said: "This is the strongest fighting we have seen in recent months. We are ducking inside our rooms."

The fighting was occurring after a surge of violence in Somalia's capital in May had killed about 200 people as insurgents battled the government and its allies. The U.N. says the conflict has displaced more than 122,000 people.

In another development Wednesday, UNICEF condemned the looting of its supplies and the continued occupation of its compound in the southern Somalia town of Jowhar by an extremist Islamic group. Looted supplies include nutritional supplements for about 40,000 children under the age of three, the agency said in a statement.

UNICEF said al-Shabab militiamen on May 17 occupied the compound when they took control of Jowhar, which had been an operational hub for the agency's humanitarian work in southern and central Somalia. The militiamen have looted supplies at least twice since taking over the compound, and storage equipment for vital vaccines for children has been destroyed, UNICEF said.

"We strongly urge that humanitarian work not be impeded in any way and demand the immediate return of our facilities in Jowhar town, as well as the release of looted equipment and supplies," said Hannan Sulieman, the agency's acting Somalia representative.

Somalia has had no effective central government for nearly two decades. The lawlessness on land also has allowed piracy to thrive off the country's coast, making Somalia the world's worst piracy hotspot.