The al-Shabab armed group has confirmed the fall of the southern town of Afmadow, one of their last remaining bases, to African Union and Somali forces.

"The Kenyan and Somali troops have now entered Afmadow. No fighting took place inside the town," Sheikh Abdiasis Abu Musab, spokesman for al-Shabab's military operation, told Reuters on Thursday.

"First we fought fiercely outside the town and then our fighters left the town as part of our tactics. However, we shall not stop fighting," he said.

Mohamud Farah, the spokesman for Somali government forces in the Juba region of the Horn of Africa country, painted a similar picture of how the town fell, seven months after Kenya sent troops into the country to battle al-Shabab.

"There were no casualties. Al-Shabab fled and no fighting took place. We are going to consolidate security for now," Farah told Reuters.

The Kenyan incursion is part of a three-pronged offensive against al-Shabab, an Islamist group that is also battling Ethiopian troops in central Somalia and an African Union force near the capital, Mogadishu.

Seizing Afmadow is a crucial step in the drive towards the southern port city of Kismayu, the hub of al-Shabab operations, about 120 km (75 miles) away.