Rebel brigades fought Hezbollah-backed forces loyal to President Bashar al-Assad in and around Syria's commercial hub of Aleppo on Sunday, trying to claw back territory lost to an assault that threatens the opposition's grip on the city, activists said.
Rebel brigades poured into Aleppo last July and have more than half the city under their control. But pro-Assad forces have deployed there in the past three weeks, suggesting a push to retake the city could be under way.
So far, Assad's forces have not made a major sweep into rebel areas, but given the size of the city, and its proximity to Turkey which supplies the opposition, it would be a major victory for the government if it were to regain Aleppo.
The battles in the city follow the capture by loyalist troops and their Lebanese Shi'ite Hezbollah guerrilla allies of Qusayr, a strategic town in central Syria, after heavy bombardment that razed much of the town.
U.N. High Commissioner for Human Rights Navi Pillay said last week she feared that the blood shed in Qusayr would be repeated in Aleppo and undermine international efforts to push for an end to the more than two year civil war.
The seizure of Qusayr restored a crucial land link between Hezbollah strongholds in Lebanon and Assad's military, which is dominated by his minority Alawite sect, an offshoot of Shi'ite Islam that has controlled Syria since the 1960s.
The involvement of Hezbollah fighters on the side of Assad, a fellow ally of the main Shi'ite power Iran, has galvanised Arab governments, including Egypt, behind the rebels, who mostly follow the Sunni version of Islam that dominates the Arab world.
Activists in the region said opposition forces, who include growing numbers of radicalised Islamists, have been mounting counter attacks on Hezbollah-backed troops and Shi'ite militiamen recruited from Shi'ite enclaves near the mostly Sunni metropolis, some 35 km (20 miles) from the border with Turkey.
Hezbollah, fighting openly in Syria to help Assad survive the uprising, does not comment on its operations in the country. A Lebanese security source said unlike Qusaiy, which is close to Hezbollah strongholds in Lebanon's Bekaa valley, the group might not send its guerrillas to unfamiliar terrain in Aleppo.
An opposition operations room in northern Aleppo said in a statement that rebel fighters had destroyed an army tank and killed 20 troops just northwest of the town of Maaret al-Arteek.
Opposition sources say rebels there have been holding back an armoured column for the last two days sent from Aleppo to re-enforce loyalists recruited from the Shi'ite villages of Nubbul and al-Zahra further to the northwest.
"Assad's forces and Hezbollah are trying to control northern rural Aleppo but they are being repelled and dealt heavy losses," Colonel Abdeljabbar al-Okeidi, a Free Syrian Army commander in Aleppo, told al-Arabiya Television.
He said Hezbollah had sent up to 2,000 fighters to Aleppo and the surrounding areas, but expressed confidence the opposition would prevail.
"Aleppo and Qusayr are different. In Qusayr we were surrounded by villages that had been occupied by Hezbollah and by loyalist areas. We did not even have a place to take our wounded. In Aleppo we have a strategic depth and logistical support and we are better organised," he said.
"Aleppo will turn into the grave of these Hezbollah devils."
Battles also raged inside Aleppo itself, where the thousands of loyalist troops and militia reinforced by Hezbollah have been massing and attacking opposition-held parts of the contested city, driving rebel fighters back.
Opposition activists and military sources said the army was also airlifting troops behind rebel lines to Ifrin, in a Kurdish area, that would give access for a bigger sweep inside the city.
"For a week the rebel forces have been generally on the retreat in Aleppo but tide has started reversing in the last two days," said Abu Abdallah, an activist in the area.
He said three main fronts had developed: inside the city; to the west at Maaret al-Arteek; and further northwest in farmland between the two Shi'ite villages and Ifrin.
The Syrian Revolution General Commission opposition group said in a statement that three people were killed in the al-Khalidiya neighbourhood of Aleppo, two by army snipers and one rebel in fighting near the airport.
It is impossible to verify the accounts because of the restrictions imposed on international media by Syria.