Syrian troops backed by Hezbollah and Iranian fighters launched an offensive south of Aleppo on Friday, expanding their 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 Islamic State (IS).
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 2 million people, is divided between the government and rebels.
"This is the promised battle," a senior government military source 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. It made no mention of Hezbollah fighters in its brief statement.
Two senior regional sources told Reuters this week that Iran has sent thousands of troops to Syria to bolster an offensive 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 IS 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.
Rami Abdulrahman, director of the UK-based monitoring group the Syrian Observatory for Human Rights, said there were heavy clashes near the Jebel Azzan region, about eight 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.
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Since Russia launched air strikes on September 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 IS 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.