The UN Security Council has added a Libyan armed group accused of involvement in the 2012 attack on the US mission in Benghazi, to its list of terrorist groups.

The council blacklisted Ansar al-Sharia on Wednesday for its ties to al-Qaeda, instating an arms embargo, assets freeze and global travel ban on the group at the request of Britain, France and the US.

The measure targets Ansar al-Sharia Benghazi and its sister group Ansar al-Sharia Derna, which both have links to al-Qaeda in the Islamic Maghreb (AQIM) and other armed groups.

In October, Ansar al-Sharia Derna pledged allegiance to the Islamic State of Iraq and the Levant which has seized control of territory in Iraq and Syria.

French Ambassador Francois Delattre said the decision would provide a boost to efforts by UN special envoy Bernardino Leon to broker a deal between Libya's many militias and the government.

"This is an important decision because it draws a clear line between, on the one hand, jihadists with whom there can be no dialogue, and on the other, those Libyan groups - Islamist and others - that must take part in talks launched by special envoy Bernardino Leon," Delattre told the AFP news agency.

British Foreign Minister Philip Hammond said "this decision sends a clear message that the international community will take action against extremist groups in Libya who pose a threat to peace and security".

"It is incumbent on all Libyans to reject these groups and all they stand for," Hammond said in a statement.

Sliding into violence

Since 2012, the Benghazi wing of Ansar al-Sharia has operated several training camps, mainly to help armed groups in Iraq and Syria and to a lesser extent in Mali, according to the request filed by the three countries.

Ansar al-Sharia Derna also took part in the September 2012 attack on the US mission and is operating camps in the northeastern Derna and Jebel Akhdar regions to train fighters for Iraq and Syria.

A third sister group, Ansar al-Sharia Tunisia, was added to the UN terrorism list in September.

French Foreign Minister Laurent Fabius told a meeting on the sidelines of the UN General Assembly in September that the group should face sanctions as part of efforts to prevent Libya from sliding further into violence.

The UN effort to broker a deal with various militias and the government led to a 12-hour humanitarian ceasefire in Benghazi earlier on Wednesday.

Libyan authorities have struggled to assert control across a country awash with weapons and powerful militias after the ousting of Muammar Gaddafi in a 2011 revolt.