AMMAN/BEIRUT: ISIS fighters advanced against rival insurgents in northern Syria Sunday, capturing areas close to a border crossing with Turkey and threatening their supply route to Aleppo city, fighters and a group monitoring the war said.

ISIS captured the town of Soran Azaz and two nearby villages after clashes with fighters from a northern rebel alliance, which was formed last December and includes both Western-backed fighters and Islamist militants.

This means ISIS will be able to move along a road leading north to the Bab al-Salam border crossing between the Syrian province of Aleppo and the Turkish province of Kilis, the anti-regime Syrian Observatory for Human Rights said.

The town’s loss is a blow to rebels grouped in the so-called Jabhat al-Shamiyya alliance (Levant Front), because the area sits on an important supply route to bring weapons into eastern Aleppo, two fighters said.

“The main supply line between Turkey and Aleppo will be severely affected,” said Abu Bakr, an alliance field commander, said in an online message.

The Levant Front was created in Aleppo in an effort to forge unity among factions in Syria that have often fought each other as well as the Syrian army and hard-line jihadi groups, undermining the revolt against President Bashar Assad.

Elsewhere, Kurdish militia wrested control of a dozen villages from ISIS either side of the jihadis’ bastion province of Raqqa Sunday, the Observatory said.

The Observatory said the Kurdish People’s Protection Units (YPG), backed by airstrikes from a U.S.-led anti-ISIS coalition, seized eight villages from jihadis on the western edges of Raqqa province.

“Kurdish units and their allies advanced and took control of at least eight villages southeast of Kobani, amid airstrikes by the international coalition,” the Britain-based monitor said.

Kurdish militia and Syrian rebels pushed ISIS back from Ain al-Arab, known in Kurdish as Kobani, which lies in northern Aleppo province on the Turkish-Syrian border, in January.

Since then, YPG units have edged east toward Raqqa, which ISIS declared the capital of its self-styled caliphate last year.

Syrian Kurdish fighters also seized control of four villages from ISIS near a border town in Hassakeh, east of Raqqa, the Observatory said.

“There are still clashes between the YPG and ISIS southwest of Ras al-Ain,” the monitor said, adding that at least eight ISIS fighters and three Kurdish militia were killed in the fighting.

ISIS Saturday launched an assault on Hassakeh’s provincial capital, advancing to within 4 kilometers of the key city.

The U.N. envoy to Syria condemned regime bombing of civilian areas as “totally unacceptable” after more than 140 people were killed in a day of heavy air raids.

In Syria, barrel bombs dropped by Assad’s helicopters killed 84 civilians, including children, in the northern province of Aleppo Saturday, a monitoring group said.“The news of aerial bombing by Syrian helicopters on a civilian area of the Aleppo neighborhood of Al-Shaar deserves the most strong international condemnation,” U.N. envoy Staffan de Mistura said in a statement.

“The use of barrel bombs must stop,” he said. “All evidence shows that the overwhelming majority of the civilian victims in the Syrian conflict have been caused by the use of such indiscriminate aerial weapons.”

He said it was “totally unacceptable that the Syrian air force attacks its own territory in an indiscriminate way, killing its own citizens.”

The Observatory said regime airstrikes throughout Syria killed at least 141 people Saturday, including 20 in a rebel-controlled village in northwest Idlib.

It said 22 people were killed in raids on the northeastern jihadi-dominated town of Shaddadi.

Its toll did not include another 43 people – including fighters from ISIS and their families – killed in a government raid on the same town.

Air raids also killed civilians in Damascus, Deir al-Zor and Deraa provinces, the monitor said.

“This is further shocking proof of the horrific and indiscriminate methods the Assad regime is using to kill and injure innocent civilians, including children,” British Foreign Secretary Philip Hammond said.

“The regime has always dropped barrel bombs in this war, but it is intensifying its strikes believing it can compensate for territorial losses,” Observatory head Rami Abdel-Rahman said.