Syria sent thousands of troops surging towards Aleppo in the early hours of Wednesday, where its forces have been pounding rebel fighters from the air, engulfing the country's largest city in total warfare to put down a revolt.
Recent days have seen Syria's 16-month-old uprising transformed from an insurgency in remote provinces into a battle for control of the two main cities, Aleppo and the slightly smaller capital, Damascus, where fighting exploded last week.
President Bashar al-Assad's forces have launched massive counter assaults in both cities. They appear to have beaten rebels back from neighbourhoods in the capital and are turning towards Aleppo, a commercial hub in the north.
Syrian forces fired artillery and rocket barrages early on Wednesday at the northern Damascus suburb of al-Tel in an attempt to seize the town from rebels, causing mass panic and forcing hundreds of families to flee the area, residents and opposition activists said.
The 216th mechanized battalion headquartered near Tel started bombarding the town of about 100,000 people at 3:15 a.m. (0015 GMT) and initial reports indicated residential apartment blocks were being hit, they said.
"Military helicopters are flying now over the town. People were awakened by the sound of explosions and are running away," Rafe Alam, one of the activists, said by phone from a hill overlooking Tel. "Electricity and telephones have been cut off."
Opposition activists said thousands of troops had withdrawn with their tanks and armoured vehicles from the strategic Jabal al-Zawiya highlands in Idlib province near the Turkish border and were headed towards Aleppo.
Rebels attacked the rear of the troops withdrawing from the region at the villages of Orom al-Joz and rami near the main Aleppo-Latakia road and at the village of al-Bara west of the Aleppo-Damascus highway, activist Abdelrahman Bakran said from the area.
In Aleppo, helicopters swirled overhead firing missiles throughout Tuesday, residents said. Rebels were battling government forces by the gates of the historic old city. Troops fired mortars and shells at rebels armed with rifles and machineguns.
"I heard at least 20 rockets fired, I think from helicopters, and also a lot of machinegun fire," a resident near one of the areas being shelled, who asked to be identified only by his first name Omar, said by telephone.
"Almost everyone has fled in panic, even my family. I have stayed to try to stop the looters; we hear they often come after an area is shelled."
Residents said fixed-wing jets had also flown over the city, followed by loud noises, although there were contradictory reports as to whether they had fired.
Some residents said they believed the planes had dropped bombs, but others said booming sounds could have been caused by supersonic jets breaking the sound barrier. A correspondent for Britain's BBC television said the jets had fired.
Assad's forces have occasionally launched air strikes from fixed-wing jets on other cities during the uprising, but tend to rely on helicopters for air strikes in urban areas.
The 16-month-old uprising has entered a new and far more violent phase in the past 10 days since rebels poured into Damascus in large numbers.
Last Wednesday, an explosion killed four members of Assad's closest circle inside a security headquarters, a blow that wiped out much of the top echelon of his military command structure and shattered the reputation for invulnerability that his family has held since his father seized power in a 1970 coup.
Western powers have been calling for Assad to be removed from power for many months, and now say they believe his days are numbered. But they fear that he will fight to the bitter end, raising the risk of sectarian warfare spreading across one of the world's most volatile regions.
Syria raised the alarm even further on Monday by confirming that it had chemical and biological weapons. In a statement that may have been intended to reassure the world but seemed to have the opposite effect, it said it would not use poison gas against rebels, only against external threats.
Assad's international protector Russia added its voice on Tuesday to those of Western countries warning him not to use chemical weapons. Western diplomats said Russia may have pressed Syria to make Monday's statement after the United States and Israel openly discussed their worries about chemical weapons.
U.S. President Barack Obama said the world would hold Assad and his entourage accountable "should they make the tragic mistake of using those (chemical) weapons".
Brigadier General Manaf Tlas, a member of Assad's inner circle who fled Syria this month, appeared on al Arabiya television in his first public comments since defecting. He called on troops to abandon the government.
"I address you ... as one of the Syrian Arab Army's sons who reject the criminal behaviour of this corrupt regime. The honourable people in the military would not accept these crimes," he said on al-Arabiya television.
Tlas is a member of the Sunni Muslim majority, and his defection was seen as a sign that the Sunni establishment had abandoned Assad, a member of the Alawite minority sect.
Elsewhere in the country, activists said government troops and pro-Assad militia known as shabbiha had attacked a mosque in a village northwest of the city of Hama.
"Troops and shabbiha left the roadblock on the edge of Shariaa and crossed the main road and began firing automatic rifles on the worshippers as they were entering the mosque," activist Jamil al-Hamwi said by telephone from the area.
"We have confirmed the names for 15 bodies and it is estimated there is a similar number still to be collected from the streets," said Hamwi, who uses a pseudonym for security reasons. The account, like others from activists, could not be confirmed. Syria restricts access by international journalists.
At least nine people were killed in army shelling of al-Herak, a town south of Deraa, the cradle of the revolt against more than four decades of Assad family rule, activists said.
Video posted on the Internet showed the shattered bodies of a veiled woman and six children in colourful pyjamas, some of them very young. Four lay on one doctor's table.
Troops Tighten Grip on Capital
In Damascus, troops were trying to snuff out rebel resistance in several areas, including Barzeh, near the centre, and the southern districts of Hajar al-Aswad, Asali and Qadam.
Tanks prowled the streets of Midan, a neighbourhood recaptured by the army from rebels on Friday.
Assad reshuffled his security team on Tuesday, according to a Lebanese security source. He said Ali Mamlouk had been named intelligence chief in place of Hisham Bekhtyar, one of four top Assad security aides killed in last week's blast.
Israel, which has publicly discussed military action to keep Syrian chemical arms or missiles out the hands of Assad's Lebanese militant allies Hezbollah, said there was no sign any such diversion had occurred.
The ferocity of the Syria conflict has only worsened, with 1,261 people killed since fighting intensified in Damascus on July 15, according to the British-based Syrian Observatory for Human Rights, which compiles reports from anti-Assad activists.
That made last week by far the bloodiest in an uprising in which activists say at least 18,000 people have been killed.
A UNHCR spokeswoman said that the number of displaced people within Syria had risen to about 1.5 million.
(Writing by Peter Graff; Editing by Michael Roddy and Stacey Joyce)