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.

Violence also erupted in Ikhlas neighbourhood near the government headquarters after rebels attacked forces loyal to Assad, who have deployed armoured vehicles, attack helicopters and increased roadblocks across the city.

Rebels fired rocket-propelled grenades at a police station in the Jdeidet Artouz area, killing at least five officers, the Syrian Observatory for Human Rights said, as the chief UN observer warned that Syria was not "on the track for peace".

Maj Gen Robert Mood, the Norwegian head of nearly 300 observers sent to the country to monitor a cease-fire that never took effect, said the mission was not working.

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 bombing

Wednesday'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 perpetrated.

Patty Culhane reports on the UN's next steps

Syrian 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".

State media initially said it was a "suicide bombing" before apparently retracting and calling it a "terrorist attack".

Assad 'losing control'

The blast came a day after the FSA - comprising army defectors and armed civilians - declared its battle to "liberate" Damascus had begun and warned the regime to "expect surprises".

White House National Security Council spokesman Tommy Vietor said Assad was "losing control," pointing to "increasing" defections and a "strengthened and more united" opposition.

Pentagon chief Leon Panetta said the international community must "bring maximum pressure on Assad to do what's right, to step down and to allow for that peaceful transition".

EU foreign policy chief Catherine Ashton said the bombing "underlines the urgent need for concerted action by the UN Security Council and the international community to enforce Kofi Annan's peace plan," referring to the UN-Arab League envoy.

Russian President Vladimir Putin agreed on Wednesday in a phone call with US President Barack Obama following the attack to work with the US to find a solution to the crisis in Syria, the White House said.

But the Kremlin stressed that "differences" remain ahead of the UN Security Council vote at 10am EDT (1400GMT) in which Russia is expected to veto a Western-drafted resolution calling for sanctions against Assad.

In-depth coverage of escalating violence across Syria

Russia earlier demanded the arrest and strict punishment of those behind what it called an "act of terror."

"We see the events as another attempt to further destabilise the situation in Syria," the foreign ministry said, calling on both sides to reassess the situation and seek peace.

Jordan's King Abdullah II warned meanwhile that his northern neighbour was on the brink of all-out civil war and that in a worst-case scenario, Syria's chemical weapons could fall into the "unfriendly" hands of al-Qaeda.

He told CNN the attack that killed core members of the Syrian regime was a "tremendous blow" to Assad but not yet the death knell for a regime that remains determined to cling to power.