On June 19, militants from the Islamic State of Iraq and Syria attacked Iranian border guards near Iran’s border city of Qasre Shirin, according to Iranian social media.

A photograph showed the bodies of at least two Iranian officers apparently killed in the skirmish. Iran’s state-controlled media didn’t initially report the clash at Qasre Shirin, as Tehran routinely censors violent border incidents.

But Iranian officials took an unusual step and eventually talked about this particular incident. The first official to react was Fath Allah Hosseini, Qasre Shirin’s representative in the Iranian parliament. Hosseini insisted that residents were not afraid of ISIS, which has captured much of northwestern Iraq in recent weeks.

Then on June 21, Brig. Gen. Ahmad Reza Pourdastan—the Iranian army’s senior ground force commander—confirmed to the state-run YJC news agency that the incident took place. But Pourdastan said that the attackers were from the Kurdish militant group Party for Free Life of Kurdistan, also known by its Kurdish acronym PEJAK.

But this an odd claim given the intensity of the attack and the location.

Iranian officers killed during June 19 clashes. Photo via Iranian social media

For one, Kurdish troops are battling ISIS forces 12 miles to the south near the Iraqi city of Khanaqin. Kurdish militia groups are scrambling to fortify their territory against further attacks. It seems unlikely they would open a second front by hitting Iran.

It’s possible that by mentioning PEJAK, Pourdastan is attempting to ease fears of an ISIS attack inside Iran. Pourdastan did not confirm Iranian casualties—a standard practice for Iranian officials.

He added that Iranian military units along Iran’s western borders are on full alert, including Iranian army aviation units equipped with AH-1 Cobra and Bell-214 Isfahan helicopters. Last week, Iranian state media also reported that the Iranian air force is on alert and ready to carry out expeditionary missions into Iraq.

Iran has previously benefited from ISIS—at least when the terror group stayed put in Syria. In 2013, when ISIS forces began attacking other rebel positions south of Aleppo, Iranian-backed forces took the opportunity to capture the city of Al Safirah, which commanded a critical supply route for Syrian dictator Bashar Al Assad’s troops.

But tensions between Tehran and ISIS have been building in recent months. Previously, ISIS announced it wouldn’t attack Iranians, in order to maintain its supply routes through Iran. But in May, amid the ongoing fighting with Al Qaeda-linked rebel groups such as Jabhat Al Nusrah and Islamic Front, ISIS retracted the assurance.

After the announcement, ISIS launched a wave of suicide attacks targeting Iranian nationals in Iraq. Last week, ISIS also began publicizing battle reports in Farsi.

As Iranian regular forces brace for a confrontation with ISIS, Iran’s special operations expeditionary unit—the Quds Force—could arm, organize and command Shia militias in Iraq.

With the attack on Qasre Shirin, it appears ISIS is striking back.

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