AMMAN / WASHINGTON / ANKARA: Western-backed Syrian rebels said Wednesday that Russian jets attacked them as they tried to advance against Iran-backed militias in a region of Syria’s southeastern desert.

They said six jets bombed their positions as they moved toward the Zaza checkpoint near Sabaa Biyar, a small town near the Damascus-Baghdad highway and the borders with Iraq and Jordan.

They identified them as Russian because they flew in formation and at higher altitude than Syrian jets.

“A sortie of Russian jets bombed us to repel our advance after we broke the first lines of defense of the Iranian militia and took over advanced positions near the Zaza checkpoint,” Saad al-Haj, a spokesman for Jaish al-Ossoud al-Sharqiya, one of the main groups in the area, told Reuters.

The southeast of the Syrian desert, known as the Badia, has become an important front in Syria’s civil war between President Bashar Assad, backed by Iran and Shiite militias, and rebels seeking to oust him.

They are competing to capture land held by Daesh (ISIS), which is retreating as it comes under intense attack in Iraq and along Syria’s Euphrates basin.

Haj said none of their fighters were killed in the Russian strike. Another Free Syrian Army official, Said Seif from the Ahmed Abdo Martyrs group, also said Russian planes hit the rebels when they began storming militia defenses.

The army and allied militias captured Zaza checkpoint and Sabaa Biyar this month to stop Western-backed FSA groups taking strategic ground vacated by Daesh.

Jets from a U.S.-led coalition against Daesh hit pro-government Iran-backed militias on May 18 as they tried to advance south of Zaza toward Al-Tanf, a base on the Syria-Jordan border where U.S. troops are training FSA rebels.

U.S. officials said the forces, which it described as being directed by Iran, posed a threat to U.S. troops and Syrian fighters it backs in the area. The militias, believed to be Shiites from Iraq, retreated to the area around Zaza checkpoint and the coalition has since warned them to stay about 50 km from the base.

Wednesday, the U.S. military said that pro-Syrian-government forces remain in the protected area despite the repeated warnings to leave.

Army Col. Ryan Dillon, spokesman for the U.S.-led coalition against Daesh in Iraq and Syria, said the forces’ continued presence is threatening U.S. and allied troops, which the coalition is prepared to defend.

Syrian troops have been on the offensive for weeks in northern, central and southern part of the country against Daesh and U.S.-backed rebels under the cover of Russian airstrikes, gaining an area almost half the size of neighboring Lebanon.

Most recently, Syrian troops and their allies have been marching toward the Daesh stronghold of Sukhneh, about 60 kilometers northeast of Palmyra.

Russians warships in the Mediterranean Sea have fired four cruise missiles at Daesh positions in Syria, the Russian Defense Ministry said Wednesday.

The ministry said that the strikes successfully hit Daesh heavy weapons and fighters from the group who had deployed and moved to Palmyra from the Daesh stronghold of Raqqa.

Moscow said it had notified the U.S., Turkish and Israeli militaries beforehand of the upcoming strike. It added that the Russian strike was promptly executed following the order, a testimony to the navy’s high readiness and capabilities.

In other developments, a U.S.-led coalition airstrike on eastern Syria has killed a founder of Daesh’s notorious Amaq propaganda agency, Syrian opposition activists said.

Rayan Mashaal, also known as Baraa Kadek, was reportedly killed in coalition bombardment on the Daesh-held town of Mayadin, near Syria’s border with Iraq, Monday.

Several Syrian activist Facebook pages, including the prominent media collective Eye on the Homeland, shared the news of his death.

“Mashaal was one of the founders of Daesh’s Amaq news agency,” Eye on the Homeland said. “He and his daughter were killed in a coalition airstrike on Mayadin in Deir al-Zor province,” the collective wrote.

A Facebook post reportedly written by Mashaal’s brother was also being circulated as confirmation of his death.

Neither the U.S.-led coalition nor the Syrian Observatory for Human Rights activist group could immediately confirm the news.

Separately, Turkey’s National Security Council said that the U.S. government’s decision to arm the Syrian Kurdish YPG militia in Syria was “not befitting” of a friend.

“It has been stressed that the policy of supporting the PKK/PYD-YPG terrorist organization, acting under the guise of the Syrian Democratic Forces, by disregarding Turkey’s expectation is not befitting of a friendship and alliance,” the council said in a statement.

Washington believes the YPG is the most effective fighting force against Daesh militants in Syria.