On June 1, 2015, US Deputy Special Presidential Envoy Brett McGurk said foreign fighters joining the Islamic State have been infiltrating into Syria through that country's border with Turkey.
In April 2014, Syria's Foreign Ministry accused Turkey of providing direct logistical and military support to terrorist groups operating in the north of the country.
"He [President Barack Obama] indicated that there's more that we would like to see Turkey do to coordinate with our broader international coalition to shut down the flow of foreign fighters to ISIL [Islamic State] into Syria."
Earnest explained Turkey has taken constructive steps since the Coalition against the Islamic State was formed, but "there is a lot more that we [the United States] believe they can and should do."
The US-led Coalition of some 60 countries has conducted more than 4,000 airstrikes on Islamic State targets in Syria and Iraq since August 2014.
At least 20,000 foreign fighters have joined the Islamic State, according to the US National Counterterrorism Center.