WASHINGTON  - The US designated three leaders of the Boko Haram militant group as terrorists Thursday in a bid to stem the violence in Nigeria. The three named by the State Department were Abubakar Shekau, widely believed to lead Boko Haram's main cell, Abubakar Adam Kambar and Khalid al-Barnawi. But the US stopped short of putting the group as a whole on its terror list. "In the last 18 months, Boko Haram or associated militants have killed more than 1,000 people," the State Department said in a statement, adding that Shekau was the most visible of the group's leaders.The two other men were accused of close links to Al-Qaeda in the Islamic Maghreb, which has been designated as a foreign terrorist organization by the United States."Under Shekau's leadership, Boko Haram has claimed responsibility for numerous attacks in northern Nigeria, its primary area of operation," the statement said, adding that most of the victims were "overwhelmingly civilian."The US designation blocks the men's "property interests subject to US jurisdiction and prohibits US persons from engaging in transactions with or for the benefit of these individuals," the statement added.Boko Haram, which means "Western education is sin" in the Hausa language spoken in northern Nigeria, is believed to have a number of factions with differing aims, including some with political links and a hardcore cell.Initially, the group said it was fighting for the creation of an Islamic state in the north of Nigeria, Africa's most populous nation and largest oil producer.But a range of demands by different people have since been issued, including the release of its members from prison.Boko Haram launched an uprising in 2009, leading to nearly a week of fighting that ended with a military assault that left some 800 people dead.The group went dormant for more than a year before reemerging in 2010 with a series of assassinations. Bomb blasts, including suicide attacks, have since become frequent and increasingly deadly.Just this week, Boko Haram claimed responsibility for suicide attacks on churches in Kaduna state on Sunday that left at least 16 people dead. They also sparked reprisals by Christian mobs against mosques and Muslims that killed dozens.Frustration over the government's inability to stop attacks by the group has triggered warnings that more residents could take the law into their own hands.Shekau was once thought to have been killed, but re-emerged in January to lead the group from the shadows.He appeared on YouTube at the time, threatening more attacks and saying Boko Haram was responsible for the January 20 violence that killed 185 people in Kano.