The U.S. backed Syrian Democratic Forces (SDF) now control approximately 40 percent of the Islamic State’s capital of Raqqa, Pentagon Spokesman Navy Capt. Jeff Davis told reporters Monday.

Davis recalled that Monday marked day 50 of the official U.S. backed push on ISIS’s capital but cautioned that SDF fighters were facing “heavy resistance.” ISIS fighters appear to be borrowing from tactics their comrades used in the last stand of the city of Mosul by focusing much of the battle in the historic part of the city.

“They are like rats. They pop up from underground and then disappear again,” an SDF fighter said of ISIS fighters in recent comments to the Associated Press. ISIS relies heavily on suicide borne car bombs to target SDF forces and have taken to posting children outside their car bomb factories in Raqqa to deter U.S. airstrikes, Operation Inherent Resolve spokesman Army Col. John Dillon told AP.

The terrorist group’s prolific use of human shields, concentration of fighters in heavily defended positions, and insurgent tactics are the same conditions which drew out U.S. backed Iraqi Security Force operations to retake the city of Mosul. The SDF do not however have the advantage of being a sovereign government with much of its own security infrastructure.

President Donald Trump’s special envoy to the U.S. anti-ISIS coalition Brett McGurk emphasized the difficulty of supporting the SDF in Syria during a State Department briefing.

“Syria is far more complicated than Iraq. It’s far more complex. As I mentioned earlier this morning, we do not have a government to work with. Raqqa is much more complicated,” he said.

U.S. military commanders refuse to a put a timeline on operations to retake Raqqa, noting that operations to retake Mosul took far longer than originally expected.

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