Sheikh Hassan Dahir Aweys, a prominent Somali opposition figure, is one of those targeted by the new sanctions.
Last year Aweys said he was working to unite his Islamic Party with al-Shabab.
Al-Shabab is battling to overthrow the US-backed Somali government. Somalia has not had an effective government for about two decades.
US authorities say they are increasingly concerned that Somalia has become a safe haven for Islamic extremists and a sanctuary for al-Qaeda-linked training camps.
Meanwhile, al-Shabab fighters are claiming to have gained full control of the city of Baidoa in southern Somalia, which they have declared to be an Islamist province.
According to reports, the group's fighters last week disarmed guards at a UN World Food Programmes (WFP) base in Wajid, near Baidoa, and also seized control of the nearby airstrip.
The WFP, however, said that its base was empty and the move would not affect its operations.