Syrian government forces say they are ready to mount a "decisive battle" for Aleppo, even as rebels say they have made gains in the city's ancient centre under intense bombardment and strafing from warplanes.
Fighting has also continued in the capital Damascus, where rebels say they are focusing on launching quick "hit-and-run" attacks against government targets, rather than taking control of districts.
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The twin fronts reflect the rising stakes for both sides in an uprising against President Bashar al-Assad's government that has so far claimed more than 18,000 lives, according to rights groups.
On Saturday, a brigade of the Free Syrian Army, the main armed opposition group, released an online video showing 48 kidnapped Iranians pilgrims who they claimed were members of that country's elite Revolutionary Guard.
The video warned that all Iranians in Syria would be "captured or killed" because of Tehran's strong backing for Assad.
Iran said those captured when their bus was commandeered on Saturday were pilgrims visiting an important Shia shrine on the outskirts of Damascus.
The pro-government al-Watan newspaper, meanwhile, has said the Syrian army is bracing itself for a "decisive battle" in Aleppo, the country's largest city.
No possible timetable was given, but activists in the city have for more than a week been saying that the government is gearing up for an all-out offensive on the northern commercial centre.
Late on Sunday, rebels clashed with the army in the city's southeastern Nayrab district, a fighter who called himself Abu Jumaa said. The army responded by shelling eastern districts.
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There were also clashes reported near the southern ring road.
Government forces pounded rebel-held areas on Sunday to keep the armed opposition from expanding their hold on the city's centre, whose loss would be a deep symbolic blow to the government
Mohammad Saeed, an anti-government activist, said that warplanes had joined in the attack by strafing rebel positions.
"Fighter jets to us are now as common as birds in the sky," Saeed said.
Saeed and the Syrian Observatory for Human Rights (SOHR), a UK-based human rights group, reported heavy shelling and clashes, mainly in districts flanking Aleppo's historic centre: Salahedin in the southwest and al-Sukkari and Hananou in the northeast.
Syria's official SANA news agency described the Aleppo rebels as "Gulf and Turkish militias" - more evidence of the deep regional fissures between Assad and allies such as Iran on one side, and nations that back the rebels on the other.
Once a busy shopping and restaurant district where residents would spend evenings with their families, the Salahedin district is now white with dust, broken concrete and rubble.
Tank shell holes gape wide on the top of buildings near the frontline, and homes of families and couples have been turned into look-outs and sniper locations for rebel fighters.
The government also claims that it has regained full control over Damascus, after driving rebels out of central districts, including near Abbassiyyin Square, on Sunday.
Khaled al-Shami, an anti-government activist in Damascus, dismissed as "nonsense" the official reports that rebels were pushed from the capital.
He said rebels are increasingly using a tactic of quick-hit attacks to frustrate security forces and keep the capital unstable.
"The Free Syrian Army does not seek to hold territory in Damascus but rather stage hit-and-run attacks that drain the regime. The rebels are present and strong there," he said.