A bomb blast which reportedly killed three senior Syrian officials, including the president's brother-in-law, has sent the country's conflict "spinning out of control", US defence minister Leon Panetta said.
Violent clashes continued overnight following the blast at the national security headquarters which is said to have killed Assef Shawkat, the president's brother-in-law and intelligence minister, defence minister Daoud Rajiha and head of crisis management office General Hassan Turkomani.
Interior minister Mohammad Ibrahim al-Shaar and National Security Bureau chief Hisham Ikhtiar were wounded in the attack.
President Bashar al-Assad has not made a public statement since the attack, fuelling rumours that the Syrian leader may have been wounded.
Following the blast, shelling continued across the capital, killing at least 214 people, including 124 civilians, according to The Syrian Observatory for Human Rights.
Syrian President Bashar Assad, left, stands next to Daoud Rajha, right
Prime Minister David Cameron called on Assad to step down on Thursday, saying: "I have a very clear message for President Assad. It is time for him to go."
The blast comes days after the Red Cross has declared the situation in Syria a civil war, meaning both sides of the conflict can be prosecuted for war crimes.
A new British-crafted Syria resolution is expected to be voted on by The United Nations later today on tougher international sanctions designed to end the 16-month conflict.
Foreign Secretary William Hague said that yesterday's bombing confirmed the "urgent need" for a chapter VII resolution of the UN Security Council on Syria. A resolution of this kind could ultimately lead to military action, but is thought likely to be vetoed by Russia.
The latest attack, for which both rebel group Liwa al-Islam (The Brigade of Islam), and the Free Syrian Army have claimed responsibility, is being touted as the most significant strike against President Bashar al-Assad since the uprising began.
Pictured in 2000, Syrian President Bashar Assad, right, his brother Maher, center, and brother-in-law Major General Assef Shawkat, left
The President’s brother in law, Assef Shawkat, the man widely regarded as the mastermind behind the government’s brutal crackdown.
The attack follows the recent defection of the country’s former ambassador to Iraq, Nawaf Fares, who fled the country, carrying with him worrying overtures of Syria’s chemical stockpile, a cache that, according to Fares, Assad would show no hesitation in using should he be corned.
A senior Republican Guard commander also recently defected, fuelling rumours that further high-profile desertions are imminent.