A massive blast struck the National Security building in Damascus on Wednesday morning, killing Syrian President Bashar Assad’s most trusted security official, his brother-in-law Assef Shawkat; Defense Minister Dawoud Rajha; and Deputy Vice President Gen. Hassan Turkmani.

As rebel forces celebrated, Damascus quickly appointed a new defense minister, Gen. Fahad Jassim Freij, who was previously the military chief of staff.

Several other senior officials were seriously wounded in the bombing, which took place as the regime’s top officials were holding security talks, and marked the most brazen rebel attack to date on the seat of government power.

Dawoud Rajha and Assef Shawkat (photo credit: AP)

Dawoud Rajha and Assef Shawkat (photo credit: AP)

Reuters said it was unclear whether Assad himself was present at the meeting.

An Israel Radio report said many Syrian troops deserted their positions in the capital after the blast. It quoted rebel sources saying troops had abandoned army vehicles and material as they fled.

State TV reported that Assad’s brother-in-law, Shawkat, who held the formal title of deputy chief of staff but who headed the president’s military intelligence hierarchy, died after being hospitalized in critical condition.

Israel Radio said Turkmani, a former defense minister, was an extremely senior and trusted official. The general was at the forefront of the battle against the rebels, it said.

Salman Shaikh, of the Brookings Institution, said Turkmani was “a Turkmen Shi’ite” who provided an “important regime link to both Ankara and Tehran.”

An amateur video posted on YouTube purported to show smoke rising from the building as the bomb detonated.

Interior Minister Muhammad al-Shaar was seriously wounded but in stable condition. Syrian intelligence chief Hisham Bekhtyar, also wounded, was undergoing surgery.

Rebel leader Riad al-Asaad said in a phone interview from his headquarters in Turkey that rebel forces planted a bomb inside a room where senior government officials were meeting.

Reuters reported that the bomber worked as a bodyguard for Assad’s inner circle. There were claims he was the defense minister’s personal bodyguard.

Initial reports had spoken of a suicide bomber. Later reports, however, appeared to confirm that a bomb had been placed in the room where the top officials met, and was detonated either by a timer or by remote control.

Government officials claimed that America and Israel were involved in the attack. Syrian Information Minister Omran Zoabi went on state TV to declare: “Harming our army and the soldiers and officers and leadership of the army, and even the honorable civilians and state institutions, [is a] hidden agenda, which is to harm Syria from within, so that Syria will bow and kneel, so to serve the American and Israeli mentality.”

The Syrian army released a statement saying its forces will continue to fight. “Whoever thinks that by targeting the country’s leaders they will be able to twist Syria’s arm is disillusioned because Syria’s people, army and leadership are now more determined than ever to fight terrorism … and cleanse the nation from the armed gangs.”

The capital has seen four straight days of clashes pitting government troops against rebels — an unprecedented challenge to government rule in the tightly controlled capital.

Israeli media monitors reported scenes of celebration in rebel strongholds. An Israel Radio analyst said the bombing — combined with key recent defections — marked a massive blow to Assad, signaling his imminent demise.

Rajha, 65, a former army general, was the most senior Christian government official in Syria. Assad appointed him to the post last year.

His death will resonate with Syria’s minority Christian population, who make up about 10 percent of Syria’s population and have generally stood by the regime.

Christians say they are particularly vulnerable to the violence sweeping the country of 22 million people, and they are fearful that Syria will become another Iraq, with Christians caught in the crossfire between rival Muslim groups.

Wednesday’s attack struck the National Security building in Damascus during a meeting of Cabinet ministers and senior security officials.

Damascus-based activist Omar al-Dimashki said Republican Guard troops surrounded the nearby al-Shami Hospital where some officials were taken for treatment.

Facing increasingly chaotic violence, the UN Security Council was scheduled to vote later Wednesday on a new resolution aimed at pressuring the Syrian regime to comply with a peace plan.

But Russia remained at loggerheads with the US and its European allies over any mention of sanctions and Chapter 7 of the UN charter, which could eventually allow the use of force to end the conflict in Syria.

Besides a government crackdown, rebel fighters are launching increasingly deadly attacks on regime targets, and several massive suicide attacks this year suggest al-Qaida or other extremists are joining the fray.

Activists say more than 17,000 people have been killed since the uprising began in March 2011.

The state-run news agency SANA reported that Wednesday’s blast was aimed at the National Security building, a headquarters for one of Syria’s intelligence branches and less than 500 meters (166 yards) from the US Embassy.

Police had cordoned off the area, and journalists were banned from approaching the site.

Earlier Wednesday, SANA said soldiers were chasing rebels in the Midan neighborhood, causing “great losses among them.” The Britain-based Syrian Observatory for Human Rights said army helicopters attacked the neighborhoods of Qaboun and Barzeh.

Diplomacy so far has failed to stop the bloodshed, and there appeared to be little hope that the UN’s most powerful body would unite behind a plan.

The key stumbling block is the Western demand for a resolution threatening non-military sanctions and tied to Chapter 7 of the United Nations Charter, which could eventually allow the use of force to end the conflict in Syria.

Russia is adamantly opposed to any mention of sanctions or Chapter 7. After Security Council consultations late Tuesday on a revised draft resolution pushed by Moscow, Russia’s deputy UN ambassador Alexander Pankin said these remain “red lines.”

Russia has said it will veto any Chapter 7 resolution, but council diplomats said there is still a possibility of last-minute negotiations.