The report by UN sanctions monitors estimates that between 3,000 and 4,000 IS jihadists were based in Libya while some of the key operatives in the extremist group were being relocated to Afghanistan.

Member-states told the monitors that the total IS membership in Iraq and Syria was "between 20,000 and 30,000 individuals, roughly equally distributed between the two countries."

"Among these is still a significant component of the many thousands of active foreign terrorist fighters," said the report.

The sanctions monitoring team submits independent reports every six months to the Security Council on the Islamic State and Al-Qaeda, which are on the UN terrorist blacklist.

IS once controlled large swathes of territory in Iraq and Syria, but last year it was driven out of Mosul and Raqa -- the twin seats of power of the Sunni extremist group.

By January 2018, IS was confined to small pockets of territory in Syria, although the report said the group "showed greater resilience" in eastern Syria.

IS "is still able to mount attacks inside Syrian territory. It does not fully control any territory in Iraq, but it remains active through sleeper cells" of agents hiding out in the desert and elsewhere, said the report.

Some member-states raised concerns about new IS cells emerging from the densely populated Rukban camp for internally displaced persons in southern Syria, on the border with Jordan, where families of IS fighters are now living.

- Relocating to Afghanistan -

The flow of foreigners leaving IS "remains lower than expected" and no other arena has emerged as a favorite destination for foreign fighters, although "significant numbers have made their way to Afghanistan", said the report.

There are an estimated 3,500-4,500 IS fighters in Afghanistan and those numbers are increasing, according to the report.

The flow of foreign fighters toward IS "has essentially come to a halt," it added.

IS finances are drying up, with one member-state estimating that its total reserves were "in the low hundreds of millions" of US dollars. Some revenue from oil fields in northeastern Syria continues to flow to IS.

IS commands only 250 to 500 members in Yemen, compared to between 6,000 and 7,000 fighters for Al-Qaeda.

In the Sahel, the Islamic State in the Greater Sahara is active mostly at the border between Mali and Niger but has less of a foothold than the Al-Qaeda-linked Jama'at Nusrat al-Islam wal-Muslimin (JMIN).

The Al-Qaeda-linked Shabaab dominates in Somalia but the report said that IS "has the strategic intent to expand to central and southern Somalia". Some Somali IS fighters may choose to relocate to Puntland, said the report.