President Bashar al-Assad's forces renewed a ground and aerial bombardment of Aleppo on Friday, extending efforts to crush rebels in Syria's commercial capital in what the United States said it feared could become a massacre.
Insurgents targeted army roadblocks and security installations, with both sides avoiding close-quarters warfare in the city of 2.5 million people, Syria's biggest urban centre.
The US State Department said credible reports of tank columns moving on Aleppo, along with air strikes by helicopters and fixed-wing aircraft, represented a serious escalation of Assad's efforts to crush a rebellion that began 16 months ago.
"This is the concern: that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," State Department spokeswoman Victoria Nuland said.
Troops stationed on the outskirts of Aleppo unleashed barrages of heavy-calibre mortar rounds on the western neighbourhoods of Saladin, al-Sukkari and al-Fardos, while Russian MI-25 helicopter gunships struck al-Sakhour in the east with rockets, several opposition activists in the city said.
In the first reported casualty on Friday, a man of about 60 wearing a traditional white prayer outfit was killed near a park in Saladin. His body was placed in a mosque pending identification.
Thirty-four people were killed in Aleppo and its environs on Thursday, according to opposition activists keeping a tally of casualties in the northern city.
"The rebels have so far been nimble, and civilians have mostly been the victims of the bombardment," said activist Abu Mohammad al-Halabi, speaking by phone from the city.
"There is lots of internal displacement, and schools have been turned to makeshift shelters that are packed. One shell hitting a school will result in a catastrophe," he said.
"The regime is massing troops and tanks at the entrances of Aleppo, but it seems it is for now content with bombarding the city, with the rebels constantly on the move."
Majed al-Nour, another activist, said rebels attacked a security outpost in the neighbourhood of Bustan al-Joz, which is close to the Aleppo city centre, on Thursday.
"The rebels are present in the east and west of the city, and have a foothold in areas of the centre. The regime forces control the entrances of Aleppo and the main thoroughfares and commercial streets and are bombarding the residential districts that fell into rebel hands," he said.
Nour said tens of thousands of people had fled Aleppo to nearby northern rural regions close to Turkey from which the Syrian army has withdrawn in recent weeks to focus on urban areas where relatively lightly armed rebels have hunkered down.
Uniting the opposition
The heavy fighting around Aleppo follows an audacious bomb attack that killed four of Assad's closest lieutenants in Damascus on July 18 and led some analysts to speculate that the government's grip was slipping.
With UN Security Council resolutions for sanctions against Syria vetoed by Russia and China for a third time last week, the United States has said it is stepping up assistance to Syria's fractured opposition, although it remains limited to non-lethal supplies such as communications gear and medical equipment.
One of the most senior figures to defect from Assad's inner circle, Brigadier General Manaf Tlas, put himself forward on Thursday as someone who could help unify the opposition inside and outside Syria on a plan for a transfer of power.
Tlas, speaking in a newspaper interview in the Saudi city of Jeddah, also said he was looking for support from Saudi Arabia and other powers. "I am discussing with ... people outside Syria to reach a consensus with those inside," Tlas told Thursday's edition of the Saudi newspaper Asharq al-Awsat.
Tlas went on to Turkey and met with foreign minister Ahmet Davutoglu. Turkey, a former ally of Assad and now one of his fiercest critics, has a heavy strategic stake in shaping any post-Assad leadership in neighbouring Syria.
Tlas appeared briefly with Davutoglu at an official guest house, but made no statement.
Turkey closed its border posts with Syria on Wednesday to all traffic except Syrian refugees.
Russia, one of the few remaining allies of the authoritarian Assad whose family has run Syria for 42 years, said calls for him to quit power were hindering efforts to end the conflict.
Foreign minister Sergei Lavrov said such calls, led by the United States, Turkey and other Western and Arab nations, were fanning violence. He reiterated Moscow's contention that support for Syrian rebel groups was tantamount to backing terrorism.
Iran promised to stand by Syria, come what may.
First vice president Mohammad Reza Rahimi told Iran's Press TV on Thursday that the Islamic Republic's support for Syria, its main Arab ally, was "unchangeable", countering suggestions that Iran could soften its backing for Assad.