The U.S. State Department will formally designate the Nigerian Islamist militant group Boko Haram as a "foreign terrorist organization" on Wednesday, congressional sources and others briefed on the matter said. 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. 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.