American-backed paramilitaries battling the Islamic State terror group in Syria came under attack by an “unknown aggressor” Friday, a day after Israeli forces launched a massive attack against pro-regime forces in the country.

Elements of the Syrian Democratic Forces or SDF, the loose-knit confederation of Arab and Kurdish militias supported by the U.S. military, came under mortar and artillery fire near the Syrian city of Deir-e-Zour, a known Islamic State redoubt.

“The hostile force engaged the SDF with artillery, and the SDF responded in self-defense, resulting in the destruction of one artillery piece,” officials from Operation Inherent Resolve said in a statement Friday. Command officials did not identify who was responsible for Friday’s skirmish, but did note U.S. commanders did reach out to their Russian counterparts “via ground and air deconfliction lines.”

Moscow and Tehran have been the only two international powers to provide military support to the Syrian regime of President Bashar Assad, during the ongoing civil war between pro-government troops and rebel factions in the country.

Friday’s attack comes amid growing tensions between the U.S., Iran, Israel and Syria, which have boiled over in recent days.

The attack in Deir-e-Zour comes a day after Israeli warplanes carried out a barrage of airstrikes against military sites in Syria tied to Iranian-backed regime forces in the country Thursday.

Those strikes were in retaliation to rocket attacks from Syria into the Golan Heights. Military officials in Tel Aviv say Iranian forces with the Islamic Revolutionary Guard Corps’ Quds Force were responsible for the Golan Heights strikes.

The Defense Department remained largely silent on the Thursday’s Israeli airstrikes into Syria, however officials inside the Pentagon remain concerned the ongoing violence between Israel and Iranian-backed regime forces in Syria could bleed over into the U.S.-led coalition against the Islamic State in the country.

U.S. Central Command chief Gen. Joseph Votel on Friday condemned the Syrian strikes by Iranian-backed forces into Israel.

“It is unacceptable. It is highly dangerous and it is raising the stakes for miscalculation and conflict in this region,” the four-star general said during a speech in Tajikistan.

During Friday’s speech, the Central Command chief laid the blame for the recent violence in Syria and Israel squarely on Tehran, saying the IRGC elements in Syria “are the principle element responsible,” for the attacks.

But a op coalition officer told reporters at the Pentagon earlier this week that the U.S. and allied commanders have not taken any additional steps to counter a potentially more aggressive Iranian presence in Syria.

“We continually reassess the threat to forces with what’s going on and we will adjust as necessary, [but] we are confident that we will retain the security of our forces operating in Iraq and Syria,” Coalition Deputy Command British Army Maj. Gen. Felix Gedney told reporters on Tuesday.

He declined to comment on what, if any, blowback U.S. or coalition troops based in Syria could face from Iranian-backed forces in Syria, in the wake of Washington’s decision to abandon the Obama-era nuclear deal with Tehran. But the one-star general did say the coalition’s mission in Syria would remain solely focused on defeating the Islamic State, regardless of other rising tensions in the region.

“Let’s be clear, our focus is absolutely on defeating ISIS. And we’re doing that by fighting alongside a partner force in the Syrian Democratic Forces, and that is enabling us to defeat ISIS in those final areas that they hold” in the country, he said.

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