A three minute video, posted online, purports to show militants in Libya pledging allegiance to Islamic State (Isis). The posting shows masked men holding rocket-propelled grenade launchers, machine guns and the Isis flag vowing to obey Daesh leader Abu Bakr Al-Baghdadi.

"We call on all Muslims to pledge allegiance to Islamic State and especially to the mujahidin to form a united force," said one fighter. The militants are said to be from the United Ajdabiya Revolutionaries' Shoura Council (UASC), a jihadist umbrella group, according to Libya Herald.

Last week, militants in Misrata also swore allegiance to Baghdadi. The Misrata Mujahidin Shura Council appealed to other groups to join with Isis, said a Sunday Times report.

IS is believed to be bolstering its presence in Libya. Recently, the Libyan interior ministry reported that around 5,000 IS militants are fighting in the country, still gripped by civil war, despite representatives of Libya's two competing parliaments signing an agreement to end the country's civil war on 17 December.

The UN confirmed reports from intelligence agencies that Libya has become Isis's fallback position, turning the country into a "back-up" caliphate.

The European Union high representative, Federica Mogherini, warned: "In Libya, there is the perfect mix ready to explode and in case it explodes, it will explode just at the gates of Europe." France's defence minister, Jean-Yves Le Drian, also issued a prediction that Libya has become the "hub" for IS to supply terrorists and weapons to affiliated groups in Algeria and Mali and to Boko Haram in Nigeria.

The possible loss of Ajdabiya would be a blow for Libya, cutting off oil ports and the gas fields that generate electricity, a move that oil expert John Hamilton says would mean "game over" for the country's economy, according to a Guardian report.