AU forces last week stormed the beaches of Kismayo to dislodge al-Shabaab from one of its last strongholds in the country. Al-Shabaab authorities last week denied losing ground, however.
Somalia is a few weeks into a new political era. Previously, there hadn't been a functioning central government in Somalia since 1991.
Residents, tribal elders and military authorities confirmed to the BBC that regional forces were in Kismayo. A Somali official said the military had taken up strategic positions at the main air and sea port in the city.
The BBC reports there was some speculation that al-Qaida's affiliate in Somalia may have pulled back as part of a trap. Parts of the city were said to be booby trapped as al-Shabaab fighters withdrew.
U.S. Secretary of State Hillary Clinton said on the sidelines of U.N. General Assembly last week that the international community needs to focus on improving the security situation in Somalia. She said it was up to the new government to establish a functional police force in order to secure political gains, however.
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