Passenger planes also won't carry "high risk" cargo or toner and ink cartridges that weigh more than one pound, according to tougher security rules put in place on Monday.
The new measures were prompted by last month's thwarted plot to send bombs hidden inside printer components to Chicago-area synagogues via cargo planes. A tip from a Saudi informant led officials in Great Britain and Dubai to safely recover two such bomb-laden packages sent from Yemen.
Bomb squads searched cargo planes in Newark and Philadelphia as well as a UPS truck in Brooklyn on October 29, while fighter jets that day also escorted a passenger jet carrying cargo from Yemen to JFK. No explosives were found.
Cargo considered "high risk" will undergo enhanced screening when headed to the U.S. on planes with no passengers, said Homeland Security Secretary Janet Napolitano.
Napolitano said the immediate ban on packages from Yemen is extended indefinitely and now includes its African neighbor across the Gulf of Aden, Somalia, which also considered a terrorist base.
In addition, international mail packages headed to the U.S. must be screened individually and come from an established postal shipper.
Federal officials are also urging cargo carriers like UPS, Federal Express and DHL to report their cargo manifests faster and to help identify which packages could be high risk, based on current intelligence.