Islamic State has greatly expanded its control over territory in Libya, and the militants are claiming to be the key defense for the North African state against foreign military intervention, according to United Nations sanctions monitors.

In their annual report to the UN security council, which was released on Wednesday, the monitors also said Libya has become more attractive to foreign fighters who mainly arrive through Sudan, Tunisia and Turkey.

The US has carried out air strikes in Libya targeting Islamic State, also known as Isis or Isil. A US air strike in the eastern city of Derna in November 2015 killed Isis’s previous leader in Libya, known as Abu Nabil.

The UN experts also said they had received information about the presence of foreign militaries in Libya supporting efforts to combat Isis, but did not name the countries as it was still investigating.

“The rise of Isil in Libya is likely to increase the level of international and regional interference, which could provoke further polarization, if not coordinated,” said the UN experts who monitor sanctions on Libya.

“In anticipation, Isil has been spreading a nationalistic narrative, portraying itself as the most important bulwark against foreign intervention,” they said.

Isis has taken advantage of a political and security vacuum following the 2011 uprising that toppled the country’s leader, Muammar Gaddafi. Western officials have estimated the number of Isis fighters to be as high as 6,000.

Late last year the UN experts said Isis had between 2,000 and 3,000 fighters. In the latest report they said “significant numbers of foreign fighters” had arrived in the Isis stronghold of Sirte.

A senior Isis militant described in an interview released by the Site monitoring group – a non-governmental counter-terrorism organization – as the new leader of the jihadists’ Libyan offshoot, said the organization is getting “stronger every day”.

The UN experts investigated whether Isis militants could use a backup of Libya’s banking system in Sirte to misappropriate funds, but all banking employees consulted said the system was either damaged or outdated.

“Consequently, control over Sirte does not give Isil access to state finances or to the wider Swift system,” the experts reported. Swift is a member-owned cooperative that banks use for account transfer requests and other secure messages.

“It is, however, likely that the site continues to hold all Libyan historic banking data, which could prove useful to anyone seeking to mask fraudulent transactions,” they said.

