Intelligence agencies who have spoken to the New York Times have said that Islamic State’s core leaders may be preparing to fall back to Sirte, Libya, the base of its most important affiliate.

They said that ISIS is asserting its control over some of the country’s oil wealth, allowing it to buy more weapons, and that Western military strategists say there are few good options to contain ISIS in Libya.

According to Western officials who spoke to the Times, as bombing starts having its effect on ISIS in Syria and Iraq, its leaders have turned toward “large-scale terrorist attacks against distant targets” – such as the Paris attacks and the downing of a Russian charter jet over Egypt; and “devoting new resources” to its outside provinces like Libya – “by far the most important”.

“The entire Islamic State government there is from abroad — they are the ones who are calling the shots,” said Nuri al-Mangoush, the head of a trucking company based in Misrata, about 65 miles west of the Islamic State’s territory around Sirte. Many of its employees live in Sirte, and five were jailed there recently.

Its base in Sirte, the port city on the Mediterranean about 300 miles southeast of Malta, a gateway to major oil fields and refineries.

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“They have made their intentions clear,” Ismail Shoukry, head of military intelligence for the region that includes Sirte, told the Wall Street Journal. “They want to take their fight to Rome.”

The Times said that “Western officials familiar with intelligence reports say it is the only affiliate now operating under the direct control of the central Islamic State’s leaders.”

According to Patrick Prior, the Defense Intelligence Agency’s top counterterrorism analyst: “Libya is the affiliate that we’re most worried about,” he said at a recent security conference in Washington. “It’s the hub from which they project across all of North Africa.”

A senior Defense Department official was quoted as saying that the United States and Britain have each sent commandos to conduct surveillance and gather intelligence on the ground. “Western intelligence agencies say they fear the core group may be preparing to fall back to Libya as an alternative base if necessary, a haven where its jihadists could continue to fight from even if it was ousted from its original territories.”

Militia leaders and Western officials estimate that the group’s forces in Libya now include as many as 2,000 fighters, with a few hundred in Sirte and many clustered to the east, around Nawfaliya. If ISIS takes Ajdabiya, it would gain control of “a strategic crossroads, vital oil terminals and oil fields south of the city.”

Residents and Western officials also told the Times that a senior ISIS leader, a former Iraqi Army officer under Saddam Hussein now known as Abu Ali al-Anbari – rumoured to be the deputy ‘Caliph’ of Islamic State – recently arrived in Libya by boat from across the Mediterranean.

Western officials said that another senior Iraqi leader of the Islamic State — Wissam Najm Abd Zayd al Zubaydi, also known as Abu Nabil — may have recently served as the group’s top commander in Libya until he was killed this month in an American airstrike near Derna.

“A great exodus of the Islamic State leadership in Syria and Iraq is now establishing itself in Libya,” Omar Adam, 34, the commander of a prominent militia based in Misrata, told the Times.

Two fuel truck drivers recently released after a month in Sirte’s main prison said their trailers were stopped on 6 October on a desert highway, and were “surprised to find themselves surrounded by two dozen masked fighters who spoke mainly in foreign dialects of Arabic — there were many Tunisians, but also Egyptians, Iraqis, Yemenis and Sudanese, they said.”

The Wall Street Journal reported that Islamic State has “called on recruits to travel to Libya instead of trying to enter Syria, while commanders have repatriated Libyan fighters from Syria and Iraq, Libyan intelligence officials said.”

Since early 2014, two rival factions have ruled Libya, effectively dividing the country. In the east, the internationally recognized government based in the town of Tobruk; in the west, an Islamist leaning government based in Tripoli that relies on Misrata fighting forces.