Somalia's Islamist group al Shabaab has stormed a military base in Lamu county in Kenya's coastal region used by both United States and Kenyan military personnel, a source and al Shabaab said, but it was unclear whether there were any casualties.The attack underscores the resilience of al Shabaab, which has maintained a campaign of deadly bomb and gun attacks despite being ejected from their bases in Somali capital Mogadishu and other areas years ago.