The bombing took place in the central Somali town of Dusamareb in the Galgudud region, 300km north of Mogadishu, the capital.
Mukhtar Robow Adumansur, a senior commander of the Union of Islamic Courts, said: "It is true that infidel planes bombed Dusamareb.
"This was an unprovoked attack.
"Aden Hashi Aryo and Sheikh Muhyadin Omar are the most important Shabaab members who were victims of this foreign aggression.
"The death of Aryo and Omar will not stop the struggle for the supremacy of Allah and the liberation of the Holy land of Somalia.
"We will continue the struggle until the Somali people are free."
Al-Shabaab broke from the Union of Islamic Courts when the latter entered into dialogue with the new government in Somalia.
The Union of Islamic Courts ruled Somalia briefly until their ousting at the end of 2006.
Al-Shabaab has fought against the Somali government and its Ethiopian allies since early 2007.
Both organistaions are on the US' list of terrorist organisations.
The US military has repeatedly attacked targets in Somalia over the past year.
They fired at least one cruise missile at alleged al-Qaeda targets in southern Somalia in March.
In June a US Navy destroyer shelled suspected al-Qaeda members in the mountainous and remote northeastern areas of Somalia.