The U.S. State Department does not officially list Nigeria's Boko Haram as a terrorist group, but has designated three prominent commanders of the militant Islamic group as international terrorists.

Abubakar Shekau, the group's top commander, along with Abubakar Adam Kambar and Khalid al-Barnawi, who are believed to have close ties to Al Qaeda in the Islamic Maghreb, have been added to the State Department's list of Specially Designated Global Terrorists.

These designations demonstrate the United States' resolve in diminishing the capacity of Boko Haram to execute violent attacks, read a statement from the State Department.

Boko Haram, whose name means Western education is sinful in the local Hausa language, has been held responsible for a wave of violent attacks against both government and civilian targets in Nigeria, including an attack on the U.N. headquarters in Abuja in August 2011 that killed 23 people.

Under Shekau's leadership, Boko Haram has carried out numerous attacks in northern Nigeria, its primary area of operation, the State Department said. In the last 18 months, Boko Haram or associated militants have killed more than 1,000 people.

The move to designate Boko Haram's top leaders as foreign terrorists comes as the State Department considers whether to designate the whole group as a Foreign Terrorist Organization.

The State Department has been hesitant to apply FTO status to Boko Haram following opposition from the Nigerian government as well as prominent U.S. academics, who have argued that its activities are primarily domestic and should not be considered an international threat.

Specifically, the Nigerian government -- although a target of Boko Haram's violence -- has protested the possible FTO designation, saying it would hinder its efforts to meet Boko Haram's grievances, which are tied in with the deeply complex set of ethnic, religious, socio-economic and political divisions in northern Nigeria.

An FTO designation would also affect Nigeria's economic ties with the U.S., making it difficult for Nigerian citizens to travel there and for Nigerian expatriates in the U.S. to send money home to their families.

The more narrowly focused designation of Boko Haram's leaders as terrorists allows the State Department to target their assets and prevent financial transactions without putting restrictions on Nigeria's economy.