"With this additional responsibility, Ambassador Jeffrey will lead and coordinate US Department of State relations with the Global Coalition to defeat (IS) and Department efforts to implement President Trump's announcement of a responsible US troop withdrawal from Syria," spokesman Robert Palladino said in a statement.

Jeffrey, a former US ambassador to Iraq and Turkey, will replace Brett McGurk as anti-IS envoy, a post from which he resigned following US President Donald Trump's controversial decision in December to withdraw American forces from Syria.

Critics say the move puts US-allied Kurdish forces at risk of attack by Turkish troops. The US-allied Kurds have played a major role in the war against IS, the jihadist group that seized swaths of territory in Iraq and Syria but has since suffered major defeats.

The December 21 announcement of McGurk's departure followed that of Jim Mattis, who quit as defense chief over fundamental disagreements with Trump, including over Syria.