Syrian troops backed by Hezbollah and Iranian fighters launched an offensive south of Aleppo on Friday, expanding the army's counter-attack against rebels across western Syria with support from Russian air strikes.
The assault means the army is now pressing insurgents on several fronts near Syria's main cities in the west, control of which would secure President Bashar al-Assad's hold on power even if the east of the country is still held by ISIS.
Aleppo, a commercial and industrial hub near the border with Turkey, was Syria's largest city before its four-year civil war, which grew out of protests against Assad's rule.
Control of the city, still home to two million people, is divided between the government and rebels.
"This is the promised battle," a senior military source in Syria said of the offensive backed by hundreds of Hezbollah and Iranian forces which he said had made some gains on the ground.
It was the first time Iranian fighters had taken part on such a scale in the Syrian conflict, he said, although their numbers were modest compared to the army force. "The main core is the Syrian army," the source said.
Hezbollah, which has supported Assad in several battles during the civil war, said the army was carrying out a "broad military operation", with support from Russian and Syrian jets, across a front at least 10 miles (15 km) wide from the southwest to southeast of Aleppo.
It made no mention of Hezbollah fighters in its statement.
Two senior regional sources said this week that Iran sent thousands of troops to Syria to bolster an offensive already underway in Hama province and ahead of the Aleppo attack.
Iran says it has sent weapons and military advisers to support its ally Assad, but has denied providing troops.
In the last week Iranian media have reported the deaths in Syria of three senior officers from Iran's Revolutionary Guards Corps. Hossein Hamedani, a corps deputy commander, was killed near Aleppo and two other officers have also died fighting ISIS forces in Syria, Iran's Tasnim news agency said.
Two senior Hezbollah officers have also been killed in Syria in the last week, a Lebanese security source said.
ARMY RETAKES VILLAGES
Rami Abdulrahman, director of UK-based monitoring group the Syrian Observatory for Human Rights, said there were heavy clashes near the Jebel Azzan region, about 12 km (8 miles) south of Aleppo city.
The area that the army and Russian jets were targeting was close to a main road heading south towards the capital Damascus, Abdulrahman said.
The army had recaptured the village of Abtin from rebel fighters, he said, as well as a tank battalion base close to Sabiqiya village. Both villages lie close to Jebel Azzan. Rebels had hit one army tank with a US-made TOW anti-tank missile.
The military source said the rebel fighters were mainly from the Islamist group Ahrar al-Sham and al Qaeda's Syrian wing, the Nusra Front, as well as the Suqour al-Sham and Failaq al-Sham insurgent groups.
The head of another rebel brigade Fursan al Haq, which is backed by Assad's foreign opponents and operates under the umbrella of the Free Syrian Army, said his fighters had sent more TOW missiles to the Aleppo area to try to stem the attack.
"The battle is ongoing, and the resistance is stronger than the attack," Fares al Bayoush, told Reuters.
Since Russia launched air strikes on Sept. 30 in support of Assad, the army has waged offensives against several insurgent-held regions in western Syria, starting with areas of Hama, Idlib and Latakia provinces taken by the rebels over the summer.
Moscow says its air campaign has targeted ISIS fighters in Syria, much like the US-led international coalition that has been separately striking the ultra-hardline Islamist group in Syria and neighbouring Iraq for over a year.
But most of the Russian air strikes appear to have targeted rival, foreign-backed insurgents whose advances in recent months, helped by supplies of the US-made TOW missiles, had threatened Assad's grip on power.
On Thursday the army targeted a long-held rebel enclave north of the city of Homs, with coordinated air strikes, artillery bombardment and ground assaults.
The Observatory said on Friday the death toll from the fighting there had risen to 60, including 30 women and children. Responding to the reports of civilian deaths on Thursday, a Syrian military source quoted by state media said Russian jets and Syrian forces do not target civilian areas.
Assad's opponents say Syrian forces have killed many thousands of civilians in the course of the war, particularly with the use of untargeted "barrel bombs" dropped from helicopters above rebel-held cities.