Between 20,000 and 30,000 Islamic State (ISIS) fighters remain in Iraq and Syria despite the jihadist group's defeat and a halt in the flow of foreigners joining its ranks, a UN report released Monday says, according to AFP.

The report by UN sanctions monitors estimates that between 3,000 and 4,000 ISIS 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 ISIS 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.

ISIS swept across Syria and Iraq in 2014, declaring a cross-border "caliphate" in territory roughly the size of Britain, attracting thousands of foreign fighters.

The jihadists suffered a series of stunningly quick defeats last year, including the loss of their de facto capital Raqqa in Syria.

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

ISIS "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.

The flow of foreigners leaving ISIS "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", the report added, according to AFP.

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

The jihadist group commands only 250 to 500 members in Yemen, compared to between 6,000 and 7,000 fighters for Al-Qaeda, it found.

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