Syria's defence minister and interior minister are among those killed after an explosion struck the National Security building in Damascus during a meeting of cabinet ministers and senior security officials, state media have reported.
Defence Minister General Rajha and his deputy, Assef Shawkat, the brother-in-law of President Bashar al-Assad, were reportedly killed on Wednesday in the deadliest assault on government officials since the violence began 16 months ago.
Also reported dead were Interior Minister Mohammed Ibrahim al-Shaar and General Hassan Turkmani, a former defence minister and currently Syria's deputy vice-president, who later died of his injuries.
The head of the national security office, Hisham Bekhtyar, was among those seriously wounded in the bombing that took place as ministers and security officials were meeting in the district of Rawda, according to state TV.
The government announced that Fahad Jassim al Feraj had already been named new defence minister.
Liwa al-Islam, an Islamist rebel group whose name means "Brigade of Islam", claimed responsibility for the blast.
The group said in a statement on its Facebook page that it "targeted the cell called the 'crisis control room' in the capital of Damascus". The attack was also claimed by the Free Syrian Army (FSA).
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"This is the volcano we talked about," FSA spokesman Qassim Saadedine said. "We have just started."
Rajha, 65, is the most senior government official to be killed in the Syrian civil war as rebels battle to oust Assad.
A former army general, Rajha was also the most senior Christian government official in Syria. Assad appointed him to the post just last year.
State media said several other participants in a top-level meeting who were wounded in the blast had been rushed to Al-Shami Hospital in the capital.
Al Jazeera's Rula Amin, reporting from Beirut, the capital of neighbouring Lebanon, said: "The fact that [the attack] happened near where the president lives is significant."
"It seems it is a very serious explosion and we are not sure if it was a suicide bomber in a car, or if it was one of the bodyguards, or one of the insiders who blew himself up as a high-level meeting was taking place with a number of ministers and high level security official attending it," our correspondent said.
The explosion came as clashes between the Syrian military and the Free Syrian Army in Damascus entered a fourth straight day.
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Fighting was also reported in the central district of al-Midan, where rebel fighters are holed up. There were no immediate reports of casualties.
Government forces and tanks were deployed in areas inside and around Damascus following the violence, activists said. Street-to-street battles were taking place across the city, sources told Al Jazeera.
"When you're talking about clashes taking place in at least seven or eight parts of Damascus, then they are affecting the entire city," said Al Jazeera's Nisreen el-Shamayleh, reporting from Amman, Jordan.
"Al-Midan is a very big Sunni neighbourhood in the heart of Damascus. Many of the other areas are also close to security installations and government offices and buildings. So these are very significant clashes taking place in the heart of Damascus," she said.
The SOHR said that more than 60 soldiers had been killed in clashes with the FSA fighters in the last 48 hours, but there was no independent confirmation of the claim.
UN Secretary-General Ban Ki-moon said on Thursday he was alarmed by the intensifying violence in Syria, saying he "strongly condemns" the bombing in Damascus.
Ban "is also gravely concerned about reports of the continued use of heavy weapons by the Syrian security forces, including in the Damascus area, against civilians, despite repeated government assurances that such weapons would be withdrawn," the United Nations said in an emailed statement.
The UN leader, who is on a three-day visit to China, urged members of the Security Council to take collective and effective action in view of the escalating situation in Syria.
"Time is of the essence," Ban said. "The Syrian people have suffered for too long. The bloodshed must end now."
The Security Council scheduled a vote for Thursday morning on a new Syria resolution after a last-minute delay on Wednesday failed to get key Western nations and Russia to agree on measures to end the dramatically escalating violence - but both sides remain deeply divided.
Britain's UN Ambassador Mark Lyall Grant said his country's Western-backed text would be put to a vote at 10am local time in New York (1400 GMT).
The document threatens non-military sanctions against President Bashar Assad's government if he doesn't withdraw troops and heavy weapons from populated areas within 10 days and is tied to Chapter 7 of the UN Charter, which could eventually allow the use of force to end the conflict.
Russia, which is a close Syrian ally, has said it will veto any Chapter 7 resolution.
Source: Al Jazeera And Agencies