US Special Envoy for Syria James Jeffrey on Saturday dubbed Iran as the "biggest challenge", Anadolu Agency reports.
"The Trump administration has made it very clear that it is taking a very strong position on pushing back on Iran," Jeffrey told a small group of journalists in Istanbul on Saturday.
"The biggest challenge at the moment is, of course, Iran. Because Daesh, […] the territorial Caliphate has been destroyed," he added.
"But Daesh poses very serious risks to Iraq […] and considerable threat to Syria. Not so much in the northeast where we are, but to the south of the Euphrates River. We're very concerned about Daesh," he added.
Last week, General Qasem Soleimani, the head of Iran's Islamic Revolutionary Guards Corps' (IRGC) elite Quds Force, was killed in a US drone strike in Baghdad.
His death marked a dramatic escalation in tensions between the US and Iran, which have often been at a fever pitch since President Donald Trump chose in 2018 to unilaterally withdraw Washington from a 2015 nuclear pact world powers struck with Tehran.
Earlier this week, Iran's IRGC launched more than a dozen ballistic missiles against US military and coalition forces in Iraq.