"We believe there has been a decrease in the number of foreign fighters traveling to the conflict," Siberell told members of the US Senate Homeland Security Committee.
Siberell explained that the numbers are based on "conclusions that come out of our intelligence community, their observations of less force strength."
Siberell explained that while the number of foreign fighters has declined within the core conflict zone of Syria and Iraq, "there has been an increase in other places," such as Libya, and the Daesh "does continue to attract new adherents and new followers."
Earlier in May, Defense Department spokesman for Operation Inherent Resolve, Col. Steve Warren, cited a decline in foreign fighters traveling to join the Islamic State had declined by 75 percent from its height.