Vladimir Voronkov also told the Security Council that the Islamic State 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 Islamic State'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 U.N. report on extremist threats from the Islamic State and al-Qaida comes as President Donald Trump has ordered a U.S. troop withdrawal in Syria, saying IS has been defeated, and a potential troop pullout in Afghanistan.
Voronkov, the undersecretary-general of the U.N. Office of Counter-Terrorism, said the threat from IS has increased because of combatants who fought with the group returning home, relocating or being released.
Michele Coninsx, head of the U.N. Counter-Terrorism Committee Executive Directorate, said IS' dramatic loss of territory "has driven its evolution into a covert and more locally focused network in Iraq, Syria and elsewhere."
She told the council the Islamic State group- also known as ISIS - aims to undermine stabilization and rebuilding in its former strongholds in Iraq and Syria.
The extremist group "remains one of the international terrorist groups most likely to carry out a large-scale, complex attack in the future," Coninsx said.
U.S. deputy ambassador Jonathan Cohen said the significant military setbacks IS 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."
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.
Cohen encouraged all countries to adapt to the changing threat from IS, praising the coalition for severely degrading its ability to raise funds and finance its operations "through destroying ISIS-controlled energy assets and removing key ISIS commanders responsible for finance."
The U.N.'s Coninsx stressed that IS, 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."
As examples, she said her experts noted "an increased use of mobile payment services by terrorist groups" in west Africa, and "the misuse of cryptocurrencies for malicious, criminal and terrorist purposes."
Russia's U.N. Ambassador Vassily Nebenzia said that while the Islamic States' income from contraband fuel continues to fall, the extremist group seized a number of oil fields in Syria's eastern province of Deir-el-Zour last autumn which enabled it to sell oil at $30-$35 a barrel for a number of months through intermediaries.
He said 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.
"Their coffers are being filled through trading in industrial products including sulfuric and phosphorous acid and cement," Nebenzia said. "They're also investing in fishing in Iraq. They're speculating on the stock markets, receiving income from cryptocurrencies, playing in online casinos, and also making use of scams on the internet and deliveries of fake medicines."
He said some fighters are also "compelled to sell the weapons and ammunition that they have."
And in a number of Afghan provinces, Nebenzia said, there has also been "active illegal mining, processing and deliveries abroad of iron, copper, gold, jewels and semi-precious stones."
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