"The end of major combat operations does not mean we have achieved the enduring defeat" of the IS group, Tillerson told a gathering of members of the coalition in Kuwait on February 13.
"ISIS remains a serious threat to the stability of the region, our homelands and other parts of the globe", he said.
"Years of experience from Afghanistan have taught us that strengthening local forces is one of our best tools in the fight against terrorism", the North Atlantic Treaty Organisation chief added.
Turkey is "an important North Atlantic Treaty Organisation ally and partner in the region", according to Tillerson, who said it faces threats within its border as well as from areas in Iraq and Syria.More news: Loris Karius 'feeling comfortable' with Virgil van Dijk in Liverpool defence
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Turkey's offensive "has detracted from our fight to defeat Islamic State in eastern Syria", Tillerson said of the ongoing offensive by US-backed Kurdish militia to clear remaining pockets of extremists.
"We will continue to be completely transparent with Turkey about our efforts in Syria to defeat ISIS, and while he gave a nod to Turkey's right to pursue its "counterterrorism efforts" on its border, maintained that the United States would "continue to train local security forces".
Turkish Foreign Minister Mevlüt Çavuşoğlu on February 14 said the US should first stress the territorial integrity of Syria.
US officials said the thrust of Tillerson's message was that "eyes have to be on the prize" and anything that hinders Islamic State's defeat will impair broader objectives such as a political transition in Syria that ultimately leads to an end of the war and blunts Iranian behavior throughout the region.
"Those who want to cooperate with us should be sensitive about a political solution and what is more important, they should stop supporting terrorist organizations that aim to divide Syria", Çavuşoğlu said in Kuwait City at a donor's conference for the reconstruction of Syria, apparently referring to the U.S.