African Union and Somali officials say troops are locked in heavy fighting with al-Shabab fighters outside the capital, Mogadishu, in a fast-moving offensive to capture a key militant stronghold.

A commander on the ground told VOA troops were advancing to the area of Elasha, the center of a sprawling camp with an estimated 400,000 refugees.

Witnesses reported heavy fighting on Wednesday and a VOA reporter said four civilians had been killed, including two children. No official casualty figures had been released.

Elasha is located about 10 kilometers west of Mogadishu and along the route leading to the town of Afgoye, the al-Shabab stronghold that pro-government troops are trying to capture.

In an interview with VOA, Somalia defense minister Hussein Arab Essa predicted troops would seized Afgoye within two days.

The area, known as the Afgoye corridor, includes camps for Somalis driven from their homes by drought and years of war.

AU and government officials said al-Shabab has been preventing aid agencies from operating in the corridor.

The U.N. Mission in Somalia has described the offensive as a “carefully planned operation” in which troops are taking every precaution to prevent harm to civilians.

The U.N. Humanitarian Coordinator for Somalia, Mark Bowden, on Wednesday reiterated his call for all parties to make efforts to minimize the impact of conflict on civilians, saying that further fighting could lead to displacement and straining of temporary settlements.

Al-Shabab has been fighting to overthrow Somalia's U.N.-backed transitional government and impose strict Islamic law.

The group once controlled most of southern and central Somalia, but has steadily lost ground in the past 18 months to AU, Somali, Kenyan and Ethiopian forces.