A statement from Benghazi-based extremist Islamic militant group Ansar al-Shariah says its leader Mohammed al-Zahawi has been killed.

The statement, posted on the group's official Twitter account Saturday, gave no details about how or when al-Zahawi was killed.

It gave condolences and vowed to take revenge and "shake the seat of power."

The group has been blamed for the September 2012 attack on the U.S. Consulate in Benghazi that killed U.S. Ambassador Chris Stevens and three other Americans. The United States designated it a terrorist organization in January 2014.

The United Nations also listed Ansar al-Shariah as a terrorist organization in November 2014, saying it runs training camps for foreign fighters travelling to Syria, Iraq and Mali.