BAB AL-HAWA, Syria/AMMAN - Syrian army helicopters pounded Damascus with rockets and heavy machine guns overnight, and tanks bombarded the capital from the ring road, to try to reverse relentless gains by rebels since much of Syrian President Bashar Assad's entourage was assassinated.The unprecedented rebel momentum of the past few days has fighters boasting that Assad's grip is being pried from the country his family has ruled since his father seized power in a coup 42 years ago. But he remains a fearsome foe.Lightly-armed rebels have been moving on foot inside residential neighborhoods and attacking security installations and roadblocks dotted across the capital.Wajeeh, a private employee who did not want to give his last name, said he saw three tanks on the southern ring road that deployed late on Friday evening and were firing at the Kfar Souseh and Mezze districts in west Damascus."The road was cut off and troops were firing mortar rounds from next to the tanks," he said.A resident of Mezzeh, a middle class district of high rise towers, villas and cactus fields, said army helicopters were striking the neighborhood with heavy machine guns and rebels were firing back "uselessly" from automatic rifles.Another resident in Barzeh, a lower middle class neighborhood in the northeast, said a barrage of mortar rounds began hitting the district before midnight and he counted six rounds hitting residential buildings.He said snipers stationed in Ush al-Wawrar, an enclave in hills overlooking Barzeh populated mainly by members of Assad's Alawite minority sect, had killed a woman earlier in the day and there were sporadic exchanged of fire between the two districts.An activist in Saida Zeinab, a poor neighborhood in the southeast of Damascus housing thousands of Sunni Muslim refugees from other parts of Syria, said it was being shelled with artillery following a helicopter rocket attack. Explosions were also heard from the suburbs of Harasta, Irbeen, Daraiya and Harran al-Awameed, where rebels fought troops on Friday.Activists said at least 100 people were killed in Damascus on Friday. Accounts could not be verified. The Syrian government restricts access by international journalists.The 'final phase'Regional and world powers are now bracing for the last phase of the conflict, hoping to wrench Assad out of power without unleashing a sectarian war that could spill across borders in one of the most volatile parts of the world.Israel said it would consider military action if needed to ensure Syrian missiles or chemical weapons do not reach Assad's allies in Lebanon, the Shi'ite Islamist movement Hezbollah."I have instructed the military to increase its intelligence preparations and prepare what is needed so that ... (if necessary) ... we will be able to consider carrying out an operation," Defense Minister Ehud Barak said.Diplomacy has failed to keep pace with events. A day after Moscow and Beijing vetoed a UN resolution that would have allowed sanctions, the Security Council approved a 30-day extension of a small, unarmed observer mission, the only outside military presence on the ground.In at least one apparent success for Assad's forces, state TV said on Friday troops had cleared the central Damascus district of Midan of "mercenaries and terrorists." It showed dead men in t-shirts, some covered in blood, others burned.Opposition activists and rebel sources confirmed on Friday that they had withdrawn from that district after coming under heavy bombardment, but said they were advancing elsewhere."It is a tactical withdrawal. We are still in Damascus," Abu Omar, a rebel commander, said by telephone.Assad's forces shelled the Abu Kamal crossing with Iraq on the Euphrates River highway, one of the most important trade routes in the Middle East, seized by rebels on Thursday.A Reuters photographer at the scene said Iraqi forces had sealed off their side of the checkpoint with concrete walls. Late on Friday explosions and gunfire could be heard from the Syrian side, which had been burned and looted.The surge in violence has trapped millions of Syrians, turned sections of the capital into ghost towns, and sent tens of thousands of refugees fleeing to neighboring Lebanon."The regime is going through its last days," Abdelbasset Seida, the leader of the main Syrian opposition umbrella group, the Syrian National Council, said in Rome, predicting a dramatic escalation in violence."The regime has been rudderless for last three days. But the aerial and ground bombardment on Damascus and its suburbs shows that it has not lost the striking force and that it is re-grouping," opposition activist Moaz al-Jahhar said by telephone from Damascus.The 16-month conflict has been transformed since Wednesday, when a bomb killed four members of the president's narrow circle of kin and lieutenants, including his powerful brother-in-law, defense minister and intelligence chief.In the days since, rebels have pushed deep into the heart of the capital and seized control of other towns. On Thursday, they captured three border crossings with Iraq and Turkey, the first time they have held sway over Syria's frontiers.At Bab al-Hawa, a busy border post with Turkey seized by advancing fighters, rebels watched on with approval while jubilant villagers looted a duty free shop, part of the vast business empire of one of Assad's cousins."This is the people's money; they are taking it back," said rebel fighter Ismail. "Whoever wants to should take it."Assad has failed to speak in public since Wednesday's blast, adding to the sense that one of the most strategically important countries in the Middle East is being torn from his grasp. A funeral was held on Friday for officials slain in the attack, but Assad did not attend and was nowhere to be seen.The next few days will determine whether Assad's government can recover from the bombing, which wiped out much of his command structure in a single blow and destroyed his clan's decades-old aura of merciless invulnerability.Rebels poured into the capital Damascus at the start of the week and have since been battling government forces in what the fighters call operation "Damascus Volcano."