The U.S. government is expected to formally apply a foreign terrorist label Thursday to three alleged leading figures of the fanatic Nigerian Muslim group Boko Haram, officials said Wednesday evening.

The action by the State and Treasury departments follows growing pressure on the administration to take stronger action against Boko Haram, Reuters reports. The group, which says it wants to establish an Islamic caliphate in northern Nigeria, has stepped up attacks on Christians and their churches.

Thursday's anticipated action, officials said, involves applying the terrorist designation to three Nigerians believedto be central figures in the group: Abubakar Shekau, aged around 43, described as a Boko Haram leader who allegedly aligned himself with al Qaeda in a video message; Abubakar Adam Kambar, aged roughly 35; and Khalid al Barnawi, aged approximately 36.

The expected action will freeze any assets they have in the United States, and bar Americans from any transactions with them.

The Rev. Ayo Oritsejafor, national president of the Christian Association of Nigeria, called Wednesday on the U.S. government to urgently list Boko Haram as a foreign terrorist organization, the Nigerian newspaper PM News reported.

Oritsejafor spoke in the wake of series of bombings in Kaduna and Yobe states that killed hundreds of Christians.

The barbaric activity of the sect is the reason we are calling on the American government to quickly designate the Boko Haram sect as a foreign terrorist organisation. The enlistment of the sect as a terrorist group is long overdue, he said.

He also denounced President Goodluck Jonathan's weakness in the face of the violence.