"We tell the American President Barack Obama to embrace Islam before we come to his country," said Fuad Mohamed "Shongole" Qalaf.
Al-Shabab has not yet launched an attack outside Africa but Western intelligence has long been worried because the group targeted young Somali-Americans for recruitment. About 20 have traveled to Somalia for training and at least three were used as suicide bombers inside Somalia.
Al-Shabab holds most of southern and central Somalia and has the support of hundreds of foreign fighters, mostly radicalized East Africans. It seeks to overthrow the weak U.N.-backed government, which is protected by 8,000 Ugandan and Burundian African Union peacekeepers.
The al-Shabab militia that killed 76 people. It has also announced its allegiance to al Qaeda and is believed to be harboring a mastermind of the twin 1998 bombings of U.S. embassies in Kenya and Tanzania that killed 224 people.
The radio message was recorded in the town of Afgoye, near the Somali capital, during a meeting of Shongole and Sheik Hassan Dahir Aweys, formerly the leader of insurgent group Hizbul Islam. The two insurgent groups had clashed several times previously but announced a merger last week. Aweys said his group will fight under al-Shabab's command.
"We have united for the sake of our ideology and we are going to redouble our efforts to remove the government and the African Union from the country," said Aweys on Monday.
In an unrelated development, the Somali information minister said the new Cabinet had approved the use of a private security contractor to train forces in the capital of Mogadishu and the program would start "soon."
Saracen International would train forces for VIP protection, said Abdulkareem Jama. He said he did not know exactly when training would start, what the men's duties would include or how many men would be trained but he said the program included the renovation of a hospital and government buildings.
Somali officials are frequently killed by insurgents, both in single assassinations and en masse in suicide bombings and attacks. The Somali ambassador in Kenya previously said that up to 1,000 men could be trained in the capital for an anti-piracy force and 300 for a presidential guard.
Saracen is already training 1,000 men for an anti-piracy force in the semiautonomous northern region of Puntland.
The program has been criticized by U.S. officials who say it is unclear who is funding it, what its objectives are and whether it breaks a U.N. arms embargo.
Jama said the Somali cabinet had discussed those issues and were satisfied the embargo was not being broken but he did not say who was funding the program.
"There is a need for training," he said. "There was an effort to slow down the project (in Mogadishu) because of those concerns."
The arid Horn of Africa nation has not had a functioning government since a socialist dictatorship collapsed in 1991. Its position on the Horn of Africa means pirates can use its long coastline to capture shipping.
Analysts fear that al Qaeda linked insurgents are also gaining ground across the Gulf of Aden in the unstable nation of Yemen. If Yemen fell, that would mean failed states on either side of the shipping route leading into the strategically vital Suez Canal, the route taken by a substantial portion of the world's oil shipments.