The United States calculates there are up to 10,000 ISIS fighters in Iraq and Syria . In the past year, U.S. officials have said they believe that more than 1,000 foreign fighters, including from Western countries, have joined ISIS. The Iraqi military began an offensive against Tikrit on Sunday , according to reports on Iraqiya TV . UN : At least 757 civilians were killed during the group 's advance in northern Iraq .
The United States calculates there are up to 10,000 ISIS fighters in Iraq and Syria . In the past year, U.S. officials have said they believe that more than 1,000 foreign fighters, including from Western countries, have joined ISIS. UN : At least 757 civilians were killed during the group 's advance in northern Iraq .