Members of Ansar al-Sharia, an al Qaeda-affiliated group, are seen near a tank taken from the army. PHOTO: REUTERS
The State Department named two groups known as Ansar al-Sharia from the eastern Libyan cities of Benghazi and Darnah and their leaders, and a third affiliated organisation from Tunisia.
Each was designated as a "foreign terrorist organisation."
Two groups named Ansar al-Sharia were set up separately in Benghazi and Darnah in 2011 after the fall of the former Libyan leader Moamer Kadhafi.
The State Department said they "have been involved in terrorist attacks against civilian targets, frequent assassinations, and attempted assassinations of security officials and political actors in eastern Libya, and the September 11, 2012 attacks against the US special mission and annex in Benghazi.
"Members of both organisations continue to pose a threat to US interests in Libya."
Ahmed Abu Khattalah, a senior leader of Ansar al-Sharia in Benghazi, and Sufian bin Qumu, the leader of the Darnah, were also both designated global terrorists.
Ambassador Chris Stevens and three other American staff were killed when heavily-armed militants overran the US mission in Benghazi, and then attacked a nearby CIA annex in a sustained, hours-long attack.
The assault roiled the 2012 presidential campaign, with Republicans accusing Democratic President Barack Obama of being lax on security and of seeking to cover up the true cause of attack.
US officials have long said that the attack was carried out by militants, but have denied that core al Qaeda leaders planned and directed the operation, as claimed by some of Obama's critics.
So far no charges have been brought and no-one has been arrested in an investigation being led by the FBI.
The third group put on the US terror list was Ansar al-Sharia in Tunisia, and its leader Seifallah Ben Hassine, commonly known as "Abu Iyadh".
It is blamed for an attack on the American school in Tunis which was set on fire and badly damaged on September 14, 2012.
The arson attack "put the lives of over one hundred United States employees in the embassy at risk," the State Department said.
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