UNITED NATIONS－The United Nations counterterrorism chief warned on Monday that recent losses by Islamic State extremists "should not lead to complacency at any level", saying the extremist group remains a global threat with up to 18,000 militants in Iraq and Syria.

Vladimir Voronkov also told the Security Council that the IS group is reported to have created a network of cells in various cities in Afghanistan, including the capital Kabul, and is closely linked to its leaders in Syria and Iraq.

He said the extremist group's "center of gravity" remains in Iraq and Syria, where it reportedly controls between 14,000 and 18,000 militants, and its central leadership maintains "an intent to generate internationally directed attacks".

His briefing on the latest UN report on extremist threats from the IS group and al-Qaida comes as US President Donald Trump has ordered a US troop withdrawal in Syria, saying the IS group has been defeated, and a potential troop pullout in Afghanistan.

Voronkov, the undersecretary-general of the UN Office of Counter-Terrorism, said the threat from the IS group has increased because combatants who fought with the group are returning home, relocating or being released.

Michele Coninsx, head of the UN Counter-Terrorism Committee Executive Directorate, said the IS group's dramatic loss of territory "has driven its evolution into a covert and more locally focused network in Iraq, Syria and elsewhere".

"Despite its dwindling control over territory that once provided it with resources and a base to plan and launch attacks, it continues to present us with many complex challenges," she told the council.

US deputy ambassador Jonathan Cohen said the significant military setbacks the IS group has suffered, notably in Iraq, Syria and the southern Philippines, are "a testament" to the work of a global coalition to defeat the extremist group.

But he said "much more work remains to be done to defeat ISIS (the IS group)".

Cohen made no mention of any troop withdrawals from Syria or Afghanistan but said "ISIS is seeking to survive, reconstitute and ultimately re-emerge in Iraq and Syria".

"ISIS is also coordinating with affiliates to plan attacks elsewhere, including Afghanistan, Southeast Asia and West Africa," he said.

The UN's Coninsx stressed that the IS group, along with other extremist groups and their affiliates, have "consistently demonstrated their intent and ability to exploit new technologies and seek innovative ways to circumvent obstacles to its financial, technical and recruitment capabilities".

Russia's UN Ambassador Vassily Nebenzia said that while the IS group's income from contraband fuel continues to fall, the extremist group seized a number of oil fields in Syria's eastern province of Deir al-Zour last autumn, enabling it to sell oil at $30-$35 a barrel for months through intermediaries.

He said the IS and other extremist groups continue to take hostages, participate in the illegal trade in drugs and agricultural products, and the sale of organs and cultural artifacts－and they are constantly looking for new sources of income.