Improvised explosive devices set off by motion detection that were used against U.S. forces in Iraq have been employed by ISIS fighters to slow the offensive against Raqqa, the so-called capital of the Islamic State in Syria, according to the U.S.-backed Syrian Democratic Forces.
ISIS has also used drones to drop munitions on the attacking forces that have surrounded the city on the Euphrates River in northeastern Syria, the SDF said. The same drone tactic was used to little effect against the Iraqi Security Forces in Mosul in northwestern Iraq.
An SCDF spokesman told Reuters earlier this month that motion-sensor triggered mines were being used by ISIS to defend key positions in Raqqa. The offensive was still proceeding to plan, the spokesman said, but the motion detector IEDs has slowed the effort to take the city that was in its sixth week.
At a Defense Department briefing Monday, Navy Capt. Jeff Davis, a Pentagon spokesman, said casualties for the SDF "absolutely" were increasing in the battle for the city. He did not give any figures.
The push against Raqqa began in the fall of 2016 with "shaping operations" to isolate the city and cut off escape routes. Throughout the offensive, NATO-ally Turkey has vehemently protested U.S. training and equipping of the SDF, whose main elements come from the Kurdish YPG, or People's Protection Units.
Turkey considers the YPG an arm of the PKK, Kurdistan Workers Party, which has been labeled a terrorist organization by the U.S.
SDF forces pushed into Raqqa from the east and west last month and cut off the southern route by taking the south bank of the Euphrates River. They captured several districts, and earlier this month they broke through the wall of the Old City of Raqqa, which is when they reportedly began encountering the motion detector IEDs.
In a report on Raqqa, the Institute for the Study of War said that the battle for the city would take several months. "The SDF has reportedly encountered intensified resistance and better-emplaced defenses over the past four weeks following initial rapid gains in districts on the outskirts," the report said.
"ISIS has extensively leveraged innovative tools to slow coalition advances, including drone-borne munitions and a new type of motion-activated IED," the report said.
In Iraq before the U.S. troop drawdown in 2011, motion-activated IEDs were sometimes used against U.S. forces. U.S. officials suspected they were supplied to the insurgents by Iran.
At the time, U.S. military officials said that the motion-activated IEDs operated on passive infrared triggers which would respond to changes in temperature, such as that created by the heat of an engine from a passing truck. Since the IEDs did not use radio frequencies as triggers, they could not be jammed electronically.