The US envoy to the coalition against Islamic State said on Wednesday that the militant group's branch in Libya is the greatest cause for concern, given its attacks in the chaotic North African country and the threat it poses to US partners such as Tunisia and Egypt.
The envoy, Brett McGurk, said in testimony to the House of Representatives Foreign Affairs Committee that the United States and its allies are making progress against the group, but face major challenges.
"Our progress will not always be linear, and we should expect setbacks and surprises," McGurk said in his prepared testimony.
McGurk also told the committee that Russian air strikes north of the key city of Aleppo were forcing US-backed Syrian opposition forces to fight against Syrian President Bashar al-Assad's forces rather than Islamic State militants.
"What Russia is doing is directly enabling ISIL," he said, using an acronym for Islamic State.