The United States on Thursday named three alleged leaders of the Nigerian militant group Boko Haram as "foreign terrorists", the first time it has blacklisted members of the Islamist group blamed for attacks across Africa's most populous nation.
The State Department identified the three as Abubakar Shekau, who it called the "most visible" leader of the group, and Abubakar Adam Kambar and Khalid al-Barnawi, who it said were tied both to Boko Haram and to al Qaeda's north African wing.
"Under Shekau's leadership, Boko Haram has claimed responsibility for numerous attacks in northern Nigeria, its primary area of operation. In the last 18 months, Boko Haram or associated militants have killed more than 1,000 people," the State Department said in an announcement.
"These designations demonstrate the United States' resolve in diminishing the capacity of Boko Haram to execute violent attacks," it said.
The action by the State and Treasury departments, first reported by Reuters on Wednesday, follows growing pressure on the Obama Administration to take stronger action against Boko Haram, which has stepped up attacks on Christian places of worship this year in its drive to establish an Islamic caliphate in northern Nigeria.
U.S. officials say the decision to list individual Boko Haram members, rather than apply the more sweeping "Foreign Terrorist Organization" label to the group as a whole as some U.S. lawmakers have demanded, reflected a desire not to elevate the group's profile.
The action freezes any assets the three men have in the United States, and bar U.S. persons from any transactions with them.
The United States has signaled growing concern over Islamist extremist groups operating in Africa, particularly al Qaeda in the Islamic Maghreb (AQIM), which has expanded its influence in the lawless Sahel region and funds operations by collecting kidnap ransoms or siphoning off the drug trade.
The United States has also sought to curb the influence of al Shabaab, the militant group which has seized control of large areas of south and central Somalia and has been blamed for attacks elsewhere in east Africa.
Both AQIM and al Shabaab are already on the official U.S. list of foreign terrorist organizations, which makes them key targets in the U.S. anti-terrorism campaign.
The State Department has been under pressure to act against Boko Haram for months.
In January, Lisa Monaco, the Justice Department's top national security official, sent a letter to the State Department arguing that the Nigerian group met the criteria for a "foreign terrorist" listing because it either engages in terrorism that threatens the United States or has a capability or intent to do so.
Boko Haram increasingly is seen as a potent threat to Nigeria, the continent's most populous state and major oil producer, and as part of growing arc of Islamist extremist groups stretching across northern Africa.
Republican senators led by Scott Brown of Massachusetts have introduced legislation requiring the State Department to determine whether Boko Haram should be designated as a terrorist group.
Republican Representative Patrick Meehan, who chairs a Homeland Security subcommittee in the House, also introduced an amendment that would force the administration to add Boko Haram to the terrorism list or explain why it was not doing so.
But a group of academic experts on Africa sent a letter to Secretary of State Hillary Clinton last month urging her not to designate Boko Haram as a terrorist group, saying such a move could backfire by enhancing the group's reputation among potential recruits and other militants.