Rebels seized control of sections of Syria's international borders and torched the main police headquarters in the heart of old Damascus, advancing relentlessly after the assassination of President Bashar al-Assad's closest lieutenants.

Syrian soldiers patrol the streets of Damascus after they regained control following a rebel bombing this week that killed four generals in President Bashar al-Assad's inner circle. Reuters

The battle for parts of the capital raged into the early hours of Friday, with corpses piled in the streets. In some neighborhoods, residents said there were signs the government's presence was diminishing.

Officials in neighboring Iraq confirmed that Syrian rebels were now in control of the Syrian side of the main Abu Kamal border checkpoint on the Euphrates River highway, one of the major trade routes across the Middle East.

Rebels also claimed control of at least two border crossings into Turkey at Bab al-Hawa and Jarablus, in what appeared to have been a coordinated campaign to seize Syria's frontiers.

In Damascus, a witness in the central old quarter district of Qanawat said the huge headquarters of the Damascus Province Police was black with smoke and abandoned after being torched and looted in a rebel attack.

"Three patrol cars came to the site and were hit by roadside bombs," said activist Abu Rateb by telephone. "I saw three bodies in one car. Others said dozens of security men and shabbiha (pro-Assad militia) lay dead or wounded along Khaled bin al-Walid street, before ambulances took them away."

The next few days will be critical in determining whether Assad's government can recover from the devastating blow of Wednesday's bombing, which wiped out much of Assad's command structure and destroyed his circle's aura of invulnerability.

Assad's powerful brother-in-law Assef Shawkat, his defense minister, Gen. Daoud Rajha and a top general were killed in Wednesday's attack. The head of intelligence and the interior minister were wounded.

Government forces have responded by blasting at rebels in their own capital with helicopter gunships and artillery stationed in the mountains overlooking it.

Assad's failure to appear in public for more than 24 hours - he was finally shown on television on Thursday swearing in a replacement for his slain defense minister - added to the sense of his power evaporating. His whereabouts are not clear.

Diplomatic efforts - rapidly overtaken by events on the ground - collapsed in disarray on Thursday when Russia and China vetoed a U.N. Security Council resolution that would have imposed sanctions unless Syrian authorities halted violence. Washington said the council had "failed utterly."

But, with violence reaching new heights in Syria, the United Nations Security Council unanimously approved a 30-day extension of the monitor mission there on Friday, throwing what amounted to a thin lifeline to Kofi Annan, the special envoy in the Syrian conflict, to save his paralyzed peace plan from total irrelevance.

Hassan Ali Turkmani, Daoud Rajha and Assef Shawkat

The 15-0 vote came only a few hours before the 300-member mission’s authorization was to expire. A failure to act would have forced the monitors into a hasty withdrawal from Syria, just as deadly mayhem, rebel advances and refugee flows from the 17-month-old uprising against the government of Assad appeared to be accelerating.

Although the work of the monitors has been suspended for more than a month because of the violence and the disregard for Mr. Annan’s plan by both President Assad’s government and his armed opponents, diplomats feared that scrapping the effort entirely would have sent a message of failure at precisely the wrong moment. Activists in Damascus said rebels were now in control of the capital's northern Barzeh district, where troops and armored vehicles had pulled out.

The army had also pulled out of the towns of Tel and Dumair north of Damascus after taking heavy losses, they said. But they said troops were hitting the western district of Mezzeh with heavy machine guns and anti-aircraft guns overnight.

The reports could not be confirmed. The Syrian government restricts access by international journalists.

A resident who toured much of Damascus late on Thursday said he saw signs the government's presence was diminishing, with only sporadic checkpoints and tanks in place in some areas. The Interior Ministry at the main Marjeh Square had a fraction of its usual contingent of guards still in place.

Shelling could be heard on the southwestern suburb of Mouadamiyeh from hills overlooking the city where the Fourth Division, commanded by Assad's brother Maher, is based, he said.

As of Friday, Syrian state TV announced that the Syrian army was able to regain control of all the neighborhoods that witnessed fighting with the rebels, while the rebels claimed that their fighters withdrew tactically from Damascus neighborhoods, promising to return as the fight for the capital continues. 

The operations to seize the border checkpoints appear to show a level of coordination and effectiveness hitherto unseen from the rebels, who have been outgunned and outnumbered by the army throughout the 16-month conflict.

Footage filmed by rebels at the Bab al-Hawa border crossing with Turkey showed them climbing onto rooftops and tearing up a poster of Assad.

Syrian intelligence chief, Hisham Bekhtyar, has died of wounds suffered in a bombing that killed three other members of Bashar al-Assad's inner circle.

"The crossing is under our control. They withdrew their armored vehicles," said a rebel fighter who would only be identified as Ali, being treated for wounds on the Turkish side.

Two officers in the rebel Free Syrian Army said fighters were keeping themselves busy into the early hours of Friday, dismantling border computer systems, seizing security records and emptying the shelves of the duty-free shop.

At least 30 government tanks in the area had not mobilized to try to recapture the border post, according to Ahmad Zaidan, a senior Free Syrian Army commander.

Officials in neighboring Lebanon said refugees were pouring across the frontier: a security source said 20,000 Syrians had crossed on Thursday.

Security Council resolution fails 

Diplomacy has been largely ineffective throughout the crisis, with Western countries condemning Assad but showing no stomach for the sort of robust intervention that saw NATO bombers help blast Libya's Muammar Gaddafi from power last year.

Thursday's failed U.N. Security Council resolution, which would have extended a small, unarmed U.N. monitoring mission, was the third that has been vetoed by Russia and China.

Syrian refugees walk with their belongings crossing the Lebanese-Syrian border at al-Masnaa. REUTERS

The vote marked the third time in nine months that Russia and China used their powers as permanent members of the 15-nation council to block resolutions on Syria. There were 11 votes in favor of the resolution. Russia and China voted against it, while South Africa and Pakistan abstained from voting. 

With the mandate for the mission set to expire on Saturday, Western states that pushed the resolution to renew the operation under a threat of sanctions against Damascus reacted angrily to the vetoes.

The U.S. ambassador to the United Nations, Susan Rice, said the Security Council had "failed utterly", and Washington would look outside the body for ways "to bring pressure to bear on the Assad regime and to deliver assistance to those in need."

Russian Foreign Minister Sergey Lavrov warned Wednesday that supporting the Syrian opposition while demanding that President Bashar al-Assad step down will lead to an impasse, according to a report from RT. 

"If the policy based on support for the opposition continues – a policy leading to an impasse, since Assad will not go on his own – and all of these mantra calls that he must step down are rooted in hopelessness,"Lavrov told reporters in Moscow.

The Russian minister warned against the temptation for outside forces to play favorites with any side in the crisis.

Lavrov’s comments came on the same day Shawkat and Rajha were killed in Damascus, which RT reported as a suicide bombing by rebel forces. 

Operation Damascus Volcano – as the rebels have dubbed their assault – has been announced, Lavrov conceded. However, it would amount to “direct support for the revolutionary movement” if the international community passes a resolution that “unilaterally forbids the government from responding (to the increasing violence)."

Moscow hopes the masterminds of the terrorist attack in Damascus will be punished.

"Moscow strongly condemns all forms and manifestations of terrorism," Foreign Ministry spokesman Alexander Lukashevich said on Wednesday. "We hope the masterminds of the terrorist attack in Damascus will be found and brought to justice."

"We see what happened as another attempt to further destabilize the situation in Syria," he added

Meanwhile, Lavrov reiterated Moscow’s firm stance that it would not support sanctions against the Syrian government.

"If it is a revolution, the United Nations should have nothing to do with that," Lavrov emphasized. "We will see as the discussions continue," but Russia cannot approve the beginning of sanctions under Chapter VII of the United Nations Charter, he added.

Moscow has proposed that Annan once again convene an action group for resolving the situation in Syria. Annan, who was in the Russian capital on Tuesday for talks with President Putin, said he welcomed the idea of further talks, Lavrov said.

"We have proposed to Kofi Annan that he once again convene the so-called action group,” the minister told reporters in Moscow.

At the same time, Lavrov repreated Moscow’s view that Iran and Saudi Arabia should also participate in the talks.

Meanwhile, Deputy Foreign Minister Gennady Gatilov commented over social media that a rise in terrorist militant activity in Syria at a time when the United Nations Security Council is discussing the situation is a dangerous trend.

"A dangerous trend: while the UN SC is discussing the settlement of the Syrian crisis, militants are intensifying terror attacks, disrupting all attempts," Gatilov wrote on Twitter on Wednesday.

China also believed the resolution to be "seriously problematic, with uneven content that is intended to put pressure on only one party,” according to a report from The Guardian. 

The country's ambassador Li Baodong said countries had made "unfounded accusations against China", adding that the resolution would "not only further aggravate the turmoil, but also result in spill over to other countries in the region."