The designation is significant because it directs U.S. law enforcement and regulatory agencies to block business and financial transactions with Boko Haram, which wants to impose Islamic law in northern Nigeria.The move makes it a crime under U.S. law to provide "material support" to the group. A State Department spokeswoman did not immediately respond to a request for comment.Boko Haram and other splinter Islamist groups are seen as the biggest security threat in Nigeria, Africa's most populous country and top oil exporter.(Additional reporting by Arshad Mohammed and Patricia Zengerle; Editing by Alistair Bell and Will Dunham)