The UN counter-terrorism chief warned Monday that recent losses by ISIS "should not lead to complacency at any level," saying the terrorist 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 ISIS’ "center of gravity" remains in Iraq and Syria, where it reportedly controls between 14,000 and 18,000 fighters. He said the ISIS "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." In Syria, fighters backed by artillery fire from a US-led coalition battled a fierce ISIS counteroffensive as they pushed to retake a last morsel of territory from the terror group. The Syrian Observatory for Human Rights said a coalition air strike killed 16 civilians including seven children trying to flee the holdout on Monday, but the US-led alliance was not immediately available for comment. More than four years after the terrorists declared a "caliphate" across large parts of Syria and neighboring Iraq, several offensives have whittled that down to a tiny holdout. The Kurdish-led Syrian Democratic Forces on Saturday announced the final push to expel hundreds of diehard extremists from that patch in eastern Syria on the Iraq border. "Heavy clashes are ongoing to pressure ISIS into surrendering," Observatory chief Rami Abdel Rahman said. SDF spokesman Mustafa Bali said the SDF responded after ISIS launched a counterattack earlier in the day. "ISIS launched a counterattack on our forces and we are now responding with rockets, air strikes and direct clashes," Bali told AFP earlier. The sound of bombs echoed dozens of kilometers (miles) away and columns of dark grey smoke could be seen from SDF territory. Bali said there were "dozens of SDF hostages held by ISIS" inside their last foothold, but denied reports of executions.