Syrian troops backed by Hezbollah and Iranian fighters launched an offensive south of Aleppo, 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 Islamic State.
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 Islamic State 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 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.
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