Intense fighting between the opposition and government forces is raging in a half-dozen areas of the Syrian capital Damascus, the day after a bomb struck at the heart of Syria’s senior command, killing at least three of President Bashar al-Assad’s top brass.Columns of black smoke rose over Damascus on Thursday as troops shelled Qaboon and Barzeh, while fighting raged in al-Midan and Zahira and loud explosions were heard in Mashrou-Dumar, said the Syrian Local Coordination Committees.Al Jazeera’s Zeina Khodr, reporting from Beirut, said many people believed the latest developments had led the Syrian conflict to a turning point."The Damascus fighting is now in its fifth day, getting close to power base of the Syrian president,” our correspondent said. "The prestige of the regime has been shattered. Losing control of Damascus [means] the regime is slowly losing its grip over the country.”Wednesday’s attack, the first to target Assad’s inner circle since a 16-month uprising erupted, came ahead of a Western showdown on Thursday with Russia and China over a draft UN resolution calling for sanctions.The explosion was blamed on a bodyguard attending a meeting of security chiefs at their headquarters, prompting the White House to say Assad was "losing control” of Syria.UN chief Ban Ki-moon and international envoy Kofi Annan called on the Security Council to take strong action, ahead of the vote on the western-backed resolution which Russia and China are expected to veto.Ban said there was an "extreme urgency” for action to make government and opposition forces halt the violence, which activists say has killed more than 17,000 people since March last year.The current 90-day UN mission in Syria ends on Friday, and if no resolution is passed by then, it would have to shut down this weekend, diplomats say.More than 200 people, mostly civilians, were killed on Wednesday, including 38 in Damascus, where rebels are pressing an all-out offensive, according to the Syrian Observatory for Human Rights.Unprecedented bombingWednesday’s bombing killed Defence Minister General Daoud Rajha, Assad’s brother-in-law Assef Shawkat and General Hassan Turkmani, head of the regime’s crisis cell on the uprising, state media said.Among those wounded were Interior Minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of National Security.Conflicting accounts have emerged of who carried out the attack on Wednesday and how it was perpetratedSyrian state media did not air any images of the blast, as in previous explosions that hit Damascus in the last two months.Assad, reported by Reuters to be in the coastal city of Latakia, made no statement on the attack, but within hours named Major General Fahad Jassim Feraj as defence minister, the state news agency SANA reported.The attack was claimed by the rebel Free Syrian Army (FSA), although another group, the Brigade of Islam, also said it was responsible.The rebels said the attack, part of Operation Damascus Volcano launched on Monday, "is the first in a series ... aimed at bringing down Assad and the pillars and symbols of the regime, whether civilian or military”.