* Islamists still control tracts of territory in south
* Al Shabaab expelled from Mogadishu in 2011
* Attacks in Mogadishu and Kenya show rebel reach
By Edmund Blair
NAIROBI, Feb 19 (Reuters) - A military offensive led by an expanded African peacekeeping force aims to drive Somali Islamist rebels out of their remaining strongholds by the end of 2014, the Somali prime minister said on ...