The United States has placed three members of the Nigerian militant group Boko Haram on its list of designated terrorists.

The U.S. State Department announced Thursday it was adding Abubakar Shekau, Abubakar Adam Kambar and Khalid al-Barnawi to the terrorist list.

A statement described Shekau as the “most visible leader” of Boko Haram. It said Kambar and al-Barnawi have close links to al-Qaida in the Islamic Maghreb, which the U.S. previously designated a terrorist organization.

The U.S. said Boko Haram and associated militants have killed more than 1,000 people in the past 18 months through attacks in Nigeria, mostly in the north.

Boko Haram has claimed responsibility for many attacks, including several church bombings, the bombing of a United Nations building, and a series of attacks in the city of Kano that killed more than 180 people.

The group has said it wants wider application of Islamic law in Nigeria, and does not recognize the government or the constitution.

Designation as a terrorist blocks individuals from holding property or assets in the U.S. and prohibits Americans from engaging in transactions with them.