ISIS commanders are changing their tactics and the group remains the biggest terrorist threat facing the world despite losing its main bases in Iraq and Syria, a UN investigator warned on Monday.

Michele Coninsx, assistant secretary general in charge of counter-terrorism, said there was evidence of the insurgents finding new ways to raise money. She also said that despite a reverse in the Middle East, its global intent and network had been retained.

Her remarks to the UN Security Council come as US-backed fighters in Syria try to clear ISIS fighters from Baghouz, the last pocket of land under the terror group's control in the north of the country.

“This change in circumstances has forced ISIS to adapt and transform itself into a covert, more locally-focused network in Iraq, yet ISIS has retained its global intent and global networks,” she said.

“Of all international terrorist organisations it remains the most likely to carry out a large-scale complex attack... and it continues its determination to undermine stabilisation efforts and to fuel sectarian tensions.”

Briefing the council on the UN secretary general's eighth report on ISIS, she said the insurgents were using mobile payment systems to raise money in West Africa and outlined concerns about blockchain technology and cryptocurrencies of becoming new channels for terrorist financing.

Her report to the council coincides with widespread concern about US President Donald Trump's decision to withdraw American troops from Syria. He said on Friday that all of ISIS's one-time caliphate would probably be back under sovereign control by the end of this week. Numerous officials have said ISIS fighters may simply have retreated temporarily and will be able to regroup as soon as coalition forces are no longer present to disrupt their operations.

The UN's latest report said the danger posed to the international community had increased partly because of foreign terrorist fighters returning to their home countries. ISIS remains active because of its residual efforts in finance, logistics, military intelligence, security, doctrine and media propaganda. And while fighters have left Syria some have simply moved into Iraq and are helping ISIS there.

“It is in a phase of transition, adaptation and consolidation,” the report said of the group. “It is organising cells at the provincial level, replicating key leadership functions.”

Speaking at Monday's meeting of the Security Council, British ambassador Jonathan Allen said ISIS was “still the most significant terrorist threat to the UK”.

“They retain their leadership and capacity to inspire and encourage others to do great harm,” he added.