Syrian rebel fighters pose for a picture in Hama
Syrian troops and armoured vehicles pushed into a rebel-held district of Aleppo on Saturday and struck back in Damascus against fighters emboldened by a bomb attack against President Bashar al-Assad's inner circle.
Opposition activists in Aleppo, Syria's biggest city and a northern commercial hub, said hundreds of families were fleeing residential areas after the military swept into the Saladin district, which had been in rebel hands for two days, reports Reuters.
Fighting was also reported in the densely-populated, poor neighbourhood of al-Sakhour.
"The sound of bombardment has been non-stop since last night. For the first time we feel Aleppo has turned into a battle zone," a housewife, who declined to be named, said by phone from the city.
The Syrian army's push in Aleppo occurred after rebels assassinated four of his top security officials this week and mounted a six-day attack in the capital that they dubbed "Damascus Volcano".
Rebels also captured three border crossings with Iraq and Turkey, and on Saturday an Iraqi security source said gunmen appeared to be taking over a fourth at Yarubiyah in Syria's Kurdish northeast.
Assad, battling a 16-month uprising against his family's four decades of autocratic rule, has not spoken in public since the assassinations, and failed to attend funeral ceremonies for his brother-in-law and two other slain officials on Friday.
A bloody crackdown on what began as a peaceful revolt has increasingly become an armed conflict between an establishment dominated by Assad's Alawite minority, an offshoot of Shi'ite Islam, and rebels drawn largely from the Sunni majority.
U.N. Secretary General Ban Ki-moon said he was sending his peacekeeping chief Herve Ladsous and top military adviser Gen. Babacar Gaye to Syria to assess the situation.
In Damascus, Assad's forces hit back overnight. Using helicopters and tanks, they aimed rockets, machineguns and mortars at pockets of lightly armed rebels moving about on foot and attacking security installations and roadblocks.
Residents said the city was quiet on Saturday morning but that heavy mortar shelling in the northeastern neighbourhood of Barzeh resumed at around 2.30 p.m. (1130 GMT). Explosions could also be heard near the southern district of Tadamon.
Most shops were closed and there was only light traffic - although more than in the past few days. Some police checkpoints, abandoned earlier in the week, were manned again.
Most petrol stations were closed, having run out of fuel, and the few that were open had huge lines of cars waiting to fill up. Residents also reported long queues at bakeries and said vegetable prices had doubled.
"I feel depressed and lonely because I have to stay indoors as there is nothing good outside. Everyone else is depressed as well," said a woman in her 50s in west Damascus who supports Assad's opponents. She declined to be identified.
An opposition activist said he had sneaked back into the Midan district, which Assad's forces seized back from rebel control on Friday, only to find his house looted.
"The doors were broken and I walked into several houses which were in the same condition," said Fadi al-Wahed. "Safes were broken into, drawers broken and furniture and television screens missing. Three army trucks were parked under the ring road flyover with loot."
The Syrian Observatory for Human Rights, an opposition group which monitors the violence in the country, said 240 people were killed across Syria on Friday, including 43 troops.
The Observatory's combined death toll over the past 48 hours stood at 550, making it the bloodiest two days of the 16-month-old uprising against Assad.
On the Iraqi-Syrian border, a security source and a separate witness said they saw gunmen in a civilian car enter the Yarubiya crossing point on the Syrian side of the frontier.
"When we contacted the Syrians there, they told us the Syrian security elements are gradually withdrawing from the place," said the security source, who works for the Iraqi customs department.
It was not immediately possible to verify the reports on the border post, but Syrian opposition activists said several towns in Syria's Kurdish northeast had passed - without a fight - into local hands in recent days as central authority eroded.
A Turkish regional governor said on Saturday Syrian rebels and "independent groups" linked to smuggling were still holding the Bab al-Hawa commercial crossing point.
Mehmet Celalettin Lekesiz said nine Turkish trucks on the Syrian side had been set on fire by the Syrian groups, contradicting statements by the rebels that they were torched by the Syrian army because of Turkey's support for the rebels.
The surge in violence has trapped millions of Syrians, turned sections of Damascus into ghost areas, and sent tens of thousands of refugees fleeing to neighbouring Lebanon.
The U.N. Security Council has approved a 30-day extension for a ceasefire observer mission, but Ban has recommended changing its focus to pursuing prospects for a political solution - effectively accepting there was no truce to monitor.
Diplomats said only half of the 300 unarmed observers would be needed for Ban's suggested plan, and several monitors were seen departing from Damascus on Saturday.
Speaking two days after Russia and China vetoed a resolution to impose further sanctions on Assad's government, Ban called on the Security Council to "redouble efforts to forge a united way forward and exercise its collective responsibility".
"The Syrian government has manifestly failed to protect civilians and the international community has collective responsibility to live up to the U.N. Charter and act on its principles," he said.
Regional and Western powers have voiced concern the conflict might become a full-blown sectarian war that could spill across borders. But Assad's opponents remain outgunned and divided.
French Foreign Minister Laurent Fabius, speaking after contacts with the head of the Arab League and Qatar's prime minister, said all three agreed that it was time for Syria's fractured opposition to prepare to take charge of the country.
"We would like to see the rapid formation of a provisional government representing the diversity of Syrian society," said Fabius. Syria's main political opposition group, the Syrian National Council, operating in exile, has so far failed to unite Assad's disparate foes on a united political platform.
On the military front, a senior Syrian defector said Assad could now rely only on an inner core of loyal army regiments, adding "the collapse of the regime is accelerating like a snowball".
General Mustafa Sheikh said Assad's forces were transporting chemical arms across Syria for possible use against the rebels.