Al-Shabaab has declared war on the African Union's peacekeeping mission in Somalia. The transitional government in Mogadishu is struggling to exert control over a country dominated by the terrorist group, an affiliate of al-Qaida.
Sources in the African Union told CNN that al-Shabaab had hundreds of fighters staged outside the Somali capital in preparation for a Ramadan assault.
Monday marks the start of the Muslim holy month.
Sources to CNN said al-Shabaab may have received a shipment of weapons from its Yemeni allies in preparation of the Ramadan assault.
A report last week from the U.S. House Committee on Homeland Security concluded there was a "looming danger" of U.S. nationals who have pledged loyalty to al-Shabaab returning to the United States to strike or help al-Qaida and its affiliates attack the United States.
U.S. lawmakers in the report expressed concern about links between al-Shabaab and al-Qaida in the Arabian Peninsula, Yemen's al-Qaida offshoot.
Somalia, meanwhile, is in the grips of lingering drought that prompted relief agencies to declare a humanitarian emergency.