In much the same way, the Islamic State's new "Caliphate" in Colonel Gaddafi's home city of Sirte now has a growing number of black African fighters alongside the Iraqis and Syrians who make up its core leadership.

An Isil supporter, believed to be Sudanese, praises the group after an execution ceremony in the Libyan city of Sirte

Colonel Muncif al Walda, a senior police officer in the nearby city of Misrata, told The Telegraph: "Illegal immigration is a menace because it brings and encourages foreign fighters to come and fight with Isil.

"Most of the migrants want to go to Europe, but some want to link up with Isil. Unfortunately, here in Libya we are right in the middle of the migration rat run."

Libyan officials spoke out as Britain and America increased their pressure on Libya's new government to accept Western military help in tackling the growing Isil threat.

Both Downing Street and the Pentagon have offered Tripoli a package that would involve up to 1,000 British troops and 5,000 Italian troops in a training role.

The Libyan government is anxious about the political impact of "Western boots on the ground", but diplomats fear that Libyan security forces - although well-versed in guerrilla fighting in war to topple Gaddafi - are not up to tackling Isil alone.

Since first establishing a foothold in Sirte a year ago, Isil is believed to have built up an army of between 2,000 and 3,000 fighters, turning the port city into a Libyan version of the group's Syrian stronghold of Raqqah.

Colonel Ismail Shukri, the head of military intelligence in Misrata, told The Telegraph that around 70 per cent of Isil's army in Sirte was made up of non-Libyans.

"The majority - I cannot tell you exactly how many - are Tunisians, while the rest are made up mostly of Sudanese, Egyptians and then people from the Sub-Saharan countries stretching from Chad and Nigeria, along with a few from Algeria and the Gulf," he said.

""Sadly, we have big open borders and long open areas, and through the routes for illegal immigration, we now have all this ideology coming through. That is one of the reasons why Isil has come to Libya."

An Isil flag painted on the front of the Ougadougou conference centre in Sirte, Libya  Photo: Sam Tarling/The Telegraph

Jamal Zubia, head of the foreign media directorate in Libya's General National Congress, added: "We hear that Isil is offering people up to $1,000 to come and fight for them. That is a lot of money in many parts of Africa."

Locals in Sirte say that sub-Saharan Africans are now a common sight among Isil's volunteers.

In April last year, a man described as Sudanese drove an explosive-laden vehicle into a checkpoint between Sirte and Misrata, killing at least six people. Pictures of other black Africans killed in action fighting for Isil in Sirte have also appeared on Libyan social media.

Others appear to play a prominent role as Isil cheerleaders. In one recent Isil video showing a homeopathist publicly beheaded for "witchcraft" in Sirte, a black African is prominent among a crowd of onlookers who clap and sing praises.

"This is what we want," he says. "This is the just law that has been missing all along."

Libyan officials say the West should do more to combat the migration problem, which they describe as one of European making. They warn that it is only a matter of time before Isil's Libya chapter makes good on its threat to despatch migrants across the Mediterranean to commit Paris-style terror atrocities.

"When some migrant who has come via Libya carries out a terror attack in Europe, the world will pay attention," Col Walda said.