It was not immediately clear if the two incidents were related but fighters involved in recent attacks in Mogadishu have warned Somalis against co-operating with the transitional government which is backed by Ethiopia.
The Islamists on Wednesday also warned against the deployment of African peacekeepers.
One Islamic leader said they will fight any foreign troops who are sent into the country.
"The UN should keep its hand off our country because the Islamic forces are ready to fight any foreign troops whether they are blue helmet or black helmet," said the insurgent leader, who asked to be identified only as Osman and claims to be part of a newly formed extremist group called the Popular Resistance Movement in the Land of the Two Migrations.
The UN hopes the 8,000-member African Union force can stabilise the country following the removal of the radical Islamic Courts movement that controlled much of southern Somalia since June.
The deployment is also meant to prevent a vacuum of power and allow the withdrawal of Ethiopian forces that supported Somalia's weak transitional government in battles against the Islamic fighters.