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The Islamic State group still may have up to 30,000 members roughly equally distributed between Syria and Iraq, and its global network poses a rising threat - as does al-Qaeda, which has an even stronger presence in some regions - a United Nations report says.

The report penned by UN experts circulated on Monday said that despite the near-defeat of IS 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 said al-Qaeda's global network also "continues to show resilience," with its affiliates and allies much stronger than IS in some spots, including Somalia, Yemen, South Asia and Africa's Sahel region.

Al-Qaeda's leaders "have grown more prominent" and have been working with the extremist group's top leader, Ayman al-Zawahiri, "projecting his authority more effectively than he could previously" including on events in Syria, the experts said.

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The report to the Security Council by experts monitoring sanctions against IS and al-Qaeda said the estimate of the current total IS 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 IS 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."

IS 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 Islamic State 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.

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The experts said the discipline imposed by IS remains intact and IS 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 IS in Syria and Iraq has come to a halt, they said, but "the reverse flow, although slower than expected, remains a serious challenge."

While the rate of terrorist attacks has fallen in Europe, the experts said some governments "assess that the underlying drivers of terrorism are all present and perhaps more acute than ever before."

This suggests that any reduction in attacks is likely to be temporary until IS recovers and reorganises and al-Qaeda "increases its international terrorist activity or other organisations emerge in the terrorist arena," they said.