Beirut - Damascus and Syria's second biggest city, Aleppo, came under shell fire on Thursday as troops loyal to President Bashar Assad stepped up efforts to crush rebels threatening the government's two main power centres.
One of the most senior figures to defect from Assad's inner circle, Brigadier General Manaf Tlas, put himself forward as someone who could help unite the fragmented opposition inside and outside Syria on a blueprint for a transfer of power.
A bomb attack that killed four of Assad's closest lieutenants last week prompted predictions among his enemies that the 46-year-old president's time in power was drawing to a close.
But in the days that have followed that attack, Assad's forces have noticeably toughened their response to the armed revolt, with fixed-wing combat aircraft seen in action over Aleppo and rebel fighters said by opposition sources to have been summarily executed on the streets of Damascus.
Residents in the capital reported a shell landing in southern districts every minute on Thursday morning. Helicopters were attacking the Hajar-al-Aswad district, one of the last rebel strongholds in the city after days of street fighting, opposition activists said.
After a major assault on rebels in Damascus last week, the army has turned to Aleppo, reinforcing troops there with an armoured column that had been operating in a northern province, apparent evidence of the government's aim not to lose control of Syria's commercial capital, a city of 2.5 million.
Fierce clashes raged in the early hours of Thursday in Aleppo, and an activist said rebels now controlled half of the city, a claim that could not be independently verified.
"There was shelling this morning on the Salaheddine and Mashhad districts," said Aleppo activist Abu Hisham. "Now it has stopped, but helicopters are buzzing overhead."
Activists said 24 people were killed in fighting in and around Aleppo on Wednesday, swelling a national death toll of about 18 000 since the revolt against Assad began 16 months ago.
In the Syrian capital, a resident in the Yarmouk Palestinian refugee camp reported heavy shelling, particularly near the southern Hajar al-Aswad district.
She said the army seemed to be targeting sites on the edges of the camp, firing shells every minute. The bombardment started around 07:00 and was still going three hours later.
As hostilities have intensified in the north, in and around Aleppo, Turkey closed its border posts to commercial traffic on Wednesday, but not to refugees fleeing Syria.
At the Syrian town of Azaz, a few kilometres south of the Turkish border, rebels appeared in control after heavy clashes over the past month in which they succeeded in driving government forces out of what had become a rubble-strewn ghost town, a Reuters correspondent who visited the town said.
Military experts believe an overstretched Syrian army is pulling back to concentrate on Aleppo and Damascus, while leaving outlying areas in the hands of rebels.
In its most recent comment on the fighting, state-run Syrian television said on Wednesday that government troops were imposing security and stability in and around Aleppo.
"The terrorists are suffering terrible losses. Groups of them are throwing their weapons away and giving themselves up. Others are fleeing for the Turkish border," the television said.
Assad himself has not spoken in public in more than a week since the Damascus bomb attack on his inner circle, confining himself to appearing at formal televised events.
General Tlas, a former friend of Assad's who could play a role in any transition of power, said he had not defected from Syria in order to play a leadership role.
Tacit Saudi support
"I am discussing with ... people outside Syria to reach a consensus with those inside," he told Thursday's edition of the Saudi newspaper Asharq al-Awsat.
"I left [Syria]...to try to help the best I can to unite the honourable people inside and outside Syria to set out a road map to get Syria out of this crisis".
Tlas, who is in Jeddah and has been interviewed by a Saudi-owned newspaper and a television station, appeared to have tacit Saudi support for his stance.
The general, a Sunni Muslim member of Assad's mostly Alawite inner circle and a senior officer in the Republican Guards, defected earlier this month.
"I will co-operate with every honourable person who wants to rebuild Syria, be it the National Council or the [rebel] Free Syria Army," he said.
Western powers have been calling for Assad to be removed from office for months, but they fear that he will fight to the end, raising the risk of sectarian warfare spreading across one of the world's most volatile regions.
UN Secretary-General Ban Ki-moon, addressing the Bosnian parliament in Sarajevo, said the world must unite to end the "slaughter" in Syria, recalling the inertia of the United Nations in 1995 during the Srebrenica massacre in Bosnia.
At the UN Security Council, members blamed each other for rising violence in Syria. Western states pledged to seek an end to the conflict outside the world body, while Russia warned of "likely catastrophic consequences" with that approach.
Russia, an ally of Syria, and China have repeatedly blocked Western-backed Security Council resolutions on Syria.