The Islamic State extremist group has around 30,000 members equally distributed between Syria and Iraq, and its global network poses a rising threat, a United Nations report says.

The report by UN experts said that despite the defeat of ISIS in Iraq and most of Syria, it is likely that a reduced “covert version” of the militant group’s “core” will survive in both countries, with significant affiliated supporters in Afghanistan, Libya, Southeast Asia and West Africa.

The experts also said that al-Qaida’s global network also "continues to show resilience", with its affiliates and allies much stronger than ISIS in some spots, including Somalia, Yemen, South Asia and Africa’s Sahel region.

The report to the Security Council by experts monitoring sanctions against ISIS and Al Qaeda said the estimate of the current total ISIS membership in Iraq and Syria came from governments it did not identify. The estimate of between 20,000 and 30,000 members includes “a significant component of the many thousands of active foreign terrorist fighters,” it said.

While many ISIS fighters, planners and commanders have been killed in fighting, and many other fighters and supporters have left the immediate conflict zone, the experts said many still remain in the two countries — some engaged militarily “and others hiding out in sympathetic communities and urban areas.”

ISIS fighters swept into Iraq in the summer of 2014, taking control of nearly a third of the country. At the height of the group’s power its self-proclaimed caliphate stretched from the edges of Aleppo in Syria to just north of the Iraqi capital, Baghdad.

With its physical caliphate largely destroyed, the movement is transforming from a “proto-state” to a covert “terrorist” network, “a process that is most advanced in Iraq” because it still controls pockets in Syria, the report said.

The experts said the discipline imposed by ISIS remains intact and its leader Abu Bakr Al Baghdadi “remains in authority” despite reports that he was injured.

“It is just more delegated than before, by necessity, to the wider network outside the conflict zone,” the experts said.

The flow of foreign fighters to ISIS in Syria and Iraq has come to a halt, they said, but “the reverse flow, although slower than expected, remains a serious challenge.”