The 2012 attacks resulted in the deaths of four U.S. personnel and became a debate flashpoint during the 2016 election.
Public demonstrations, a Libyan military strongman and defections to ISIS have all damaged Ansar al-Sharia's standing in Libya.
Authorities investigating the Manchester Arena explosion are likely to probe the bomber's links to Libya, where he spent three weeks before returning to the U.K. to carry out the attack.
Facing military defeats in Iraq and Syria, the Islamic State group may be preparing a mass retreat to war-torn Libya.
The forces posted pictures of desert hideouts covered with sand and grass and said the sites were being used for training. Shells, suicide belts and booby traps had been recovered there, the statement added.
British-led intervention contributed to the growth of ISIS in Libya, a parliamentary report found.
A Libyan militia has claimed it shot down a helicopter carrying French special forces on Sunday.
The Libyan city is ISIS's most important stronghold in the country.
The coastal city is the last ISIS stronghold in Libya.
Countries including the U.S. and Russia say they will support exemptions to a U.N. arms embargo.
But other rebels say they won't be so welcoming to the new regime.
Around 20 Kenyans have traveled to join ISIS in Libya and the Middle East.
Tunisia has built a 125-mile barrier along its border with lawless Libya.
Representatives of the Tobruk and Tripoli parliaments have proposed an initial deal to form a national government.