BEIRUT - Damascus and Syria's second biggest city, Aleppo, came under shell fire on Thursday as troops loyal to President Bashar al-Assad stepped up efforts to crush rebels threatening the government's two main power centers. 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 armored 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 7 a.m. (0400 GMT) 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 miles 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.
