The town of Arsal, which is on the Lebanese side of the border Photo: Alessio Romenzi/The Telegraph
The group has been training new recruits and defectors from smaller rebel factions in Qalamoun, a militarily strategically important province in the south-west of Syria that borders Lebanon.
Several of those smaller rebel groupings, some aligned with the more moderate “Free Syrian Army”, have capitulated to the jihadists in recent months with many of their fighters joining Isil.
The growth of the group in the area means Sunni Isil fighters in Syria are now at the edge of the Lebanese heartland of its Shia arch enemy Hizbollah, whose men are fighting alongside the regime of Bashar al-Assad.
“The moderate rebel groups on the border have collapsed, and their men have joined Isil,” said Ahmed Flity, the deputy mayor of Arsal, a Lebanese border town that has effectively been cut off from the rest of the country by security forces, because of the threat from jihadists in the area.
The black flag bearing the Isil logo was clearly visible fluttering only a few hundred yards from the lone Lebanese army checkpoint marking the border between Arsal and Syria when the Telegraph visited last week.
This was once the principal route for smuggling money and satellite phones to Syrian activists opposed to Bashar al-Assad, the Syrian president, and later to moderate rebel fighters. This newspaper watched the jihadists move with confidence around the rocky mountainous terrain.
They bought weapons and refuelled their trucks with black-market oil sold by smugglers who have set up shop in this no-man’s land, far from the reach of any country’s laws.
Abbas Ibrahim, the head of Lebanon’s General Security office, has estimated that as many as “700” fighters from less extreme groups have “pledged allegiance” to Isil, swelling its ranks to over 1000 men…