There appeared to be no immediate contact from the kidnappers.
One witness said that three aid workers were seized, along with a Somali translator who was later released.
Blalock said she could not confirm that a translator had been release, nor the nationalities of those captured.
The WFP in Nairobi said they were looking into the reports.
She said that earlier reports that two of the abductees were a Russian and a Ugandan were incorrect.
According to the UN, 35 aid workers were killed in Somalia in 2008 and 26 were abducted.
Two workers have been killed this year.