Sources in Libya say French special forces are among those working against Islamic State in the country. A small French detachment has been operating from Benghazi’s Benina airport, the sources have reported, assisting forces of the internationally-backed Libyan authorities in Tobruk.

Le Monde said special forces units, alongside the DGSE, France’s external security directorate, have been in Libya for several months, and coordinated the November US strike on Derna which killed the most senior Isis leader in the country, Iraqi Abu Nabil al-Anbari. The Pentagon has confirmed that US forces were deployed there in January.

At a foreign affairs select committee hearings in London earlier this month, UK Foreign Office minister Tobias Ellwood declined to comment on British special forces operations, but said the RAF is flying missions over the country in preparation for possible attacks on IS. Italy announced on Monday that it would allow armed US drones to strike Libyan targets from bases in Sicily.

Isis fighters stormed the centre of the coastal town of Sabratha on Tuesday, four days after 41 of their number were killed in a US bombing raid, battling through the night with local militias. The police station was overrun, and when it was recaptured militia forces found the bodies of 11 beheaded police officers.

Fighting continued at daybreak after battles around Sabratha’s hospital and football stadium, with Isis fighters redeploying to a western suburb of the city, which lies 30 miles from Tripoli.

The attack underlines the problems an evolving US-led coalition faces in trying to contain a rapidly expanding Isis. Hours before the Sabratha fighting, America’s special envoy for combatting Isis, Brett McGurk, told reporters in Washington that the terrorist group have now prioritised Libya for recruiting, saying: “It’s trying to attract as many foreign fighters to Libya as possible.”

Analysts say Isis fighters are arriving from Tunisia and sub-Saharan Africa, mingling with thousands of migrants crossing the Sahara seeking boats to Europe.

“Isis is creating a real African jihadist army, we can see mass arrivals of jihadis, they are impossible to control for the simple reason that they use the same route as migrants,” said Paris-based terrorism expert David Thomson. “The airstrikes can reduce the shock that is coming, but they need ground troops to stop it.”