The U.N. counter-terrorism chief is warning that recent losses by Islamic State extremists "should not lead to complacency at any level," saying the extremist group remains a global threat.

Vladimir Voronkov told the Security Council Monday that the threat is increased by combatants who fought with the group returning home, relocating or being released.

He said the Islamic State group's "center of gravity" remains in Iraq and Syria, where it reportedly controls between 14,000 and 18,000 militants.

He said the IS "has continued to evolve into a covert network operating at the local level and organizing itself at the provincial level, with a reported intent to undermine any form of stabilization on the ground."

He said its central leadership "retains an influence and maintains an intent to generate internationally-directed attacks."