The Libyan militant group Ansar al-Sharia, which is linked to al-Qaeda and deemed a terrorist organization by the UN and United States, announced its "dissolution" in a communique published online on Saturday.
Washington accuses the group of being behind the September 11, 2012 attack on the US consulate in the eastern city of Benghazi in which ambassador Christopher Stevens and three other Americans were killed.
Ansar al-Sharia is one of the militant groups that sprung up in Benghazi, Libya's second largest city, in the chaos following the death of dictator Muammar Qaddafi in 2011. They overran the city in 2014.
East Libyan military strongman Khalifa Haftar earlier this month launched an offensive to oust militant fighters from their two remaining strongholds in Benghazi.
In its communique Ansar al-Sharia said it had been "weakened" by the fighting.
The group lost its leader, Mohammed Azahawi, in clashes with Haftar's forces in Benghazi at the end of 2014.
Most of its members then defected to ISIS. Ansar al-Sharia later joined the Revolutionary Shura Council of Benghazi, a local alliance of Islamist militias.
At its zenith, Ansar al-Sharia was present in Benghazi and Derna in eastern Syria, with offshoots in Sirte and Sabratha, western Libya.
The organization took over barracks and other sites abandoned by the ousted Qaddafi forces and transformed them into training grounds for hundreds of militants seeking to head to Iraq or Syria.