AMMAN/BEIRUT: Violence across Syria claimed the lives of at least 250 people Thursday, the highest since the beginning of the revolt against President Bashar Assad’s regime, according to the Syrian Observatory for Human Rights.

Syrian rebels battled deep into the heart of Damascus against government troops, who unleashed artillery and helicopter gunships on their own capital in retaliation for the assassination of President Bashar Assad’s closest security officials.

Diplomatic efforts that have been long overtaken by events on the ground collapsed in disarray when Russia and China vetoed a U.N. Security Council resolution that would have imposed sanctions on Syrian authorities unless they stopped using heavy weapons. Washington said the Council had “failed utterly.”

At least 93 of those killed Thursday were government troops, the opposition monitoring group said.

It added that it was still gathering information on the number of rebel fighters who had been killed from sources on the ground and expected the overall death toll to rise significantly.

The stakes in the 16-month conflict – already in a climactic phase with rebels battling their way into the center of the capital – were raised Wednesday by a bomb that killed top members of Assad’s inner circle. Assad’s powerful brother-in-law, his defense minister and a top general were killed, while the head of intelligence and the interior minister were wounded in the attack on a crisis meeting inside a security headquarters.

Assad did not appear in public in the 24 hours after the bombing, prompting a flurry of rumors that he had fled the capital to coastal Lattakia. He eventually appeared Thursday on television in images taken at the swearing in of a replacement for his slain defense minister at the Presidential Palace in Damascus. State television said Assad had wished the new defense minister good luck, but he has yet to make any direct address to the country following the blast.

Residents said heavy shelling by security forces and firing from helicopters went on through the night and continued Thursday in Damascus. Some reported explosions in the capital’s troubled northeastern and southern districts.

A witness said rebels had attacked the main police headquarters in Damascus. “Gunfire has been intense for the past hour. It is now dying down, but the streets around the police command remain empty,” said a resident of Qanawat, an old central district where the Damascus Province police headquarters is located.

Other parts of the city were paralyzed. Homes and shops were shuttered as Damascenes feared violence.

International diplomacy has been ineffective throughout the crisis, with Western countries condemning Assad but lacking the stomach for the kind of intervention that saw NATO bombers help blast Libya’s Moammar Gadhafi 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.

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.”

“The United States has not, and will not, pin its policy on an unarmed observer mission that is deployed in the midst of such widespread violence and that cannot count on even the most minimal support of this Security Council.”

Britain proposed a new four-paragraph resolution that would extend the mission for another 30 days that could be voted on later Thursday, diplomats said.

The fate of the monitoring mission remains unclear with its mandate expiring in two days. Kofi Annan, the international mediator behind a peace plan, said he was disappointed.

Chief of the mission Major General Robert Mood left his hotel in Damascus Thursday to fly to Geneva, saying Syria was not on track for peace and violence was escalating. “It pains me to say, but we are not on the track for peace in Syria, and the escalations we have witnessed in Damascus over the past few days is a testimony to that,” Mood said in a statement to reporters.

“I expressed my condemnation of the attack yesterday to the Syria government. I call on the parties to end the bloodshed and violence in all its forms, and recommit to a peaceful solution to this conflict,” he said.

Most attention is now on the streets. Experts say the next few days will be critical in showing how well the government can recover from the double blow of losing its top security officials and their image of untouchable might.

“Everyone is looking now at how well Assad can maintain the command structure. The killings yesterday were a huge blow, but not fatal,” said a Western diplomat following Syria.

“I can’t even tell you what is going on outside because I’ve shuttered the windows and locked the doors. I just hear every now and then the gunfire, it’s like it’s in the room,” said a resident near Midan, reached by telephone.

“Everyone in the neighborhood is arming themselves. Some with machine guns, some with shotguns. Some even just with knives,” said another near Al-Midan.

The bombing Wednesday seemed part of a coordinated assault on the capital that has escalated since the start of the week.

Rebels call it the “liberation of Damascus” after months of fierce clashes that activists say have killed 17,000 people.

U.N. chief Ban Ki-moon condemned Wednesday’s blast and expressed grave concern over the use of heavy weapons. “Time is of the essence,” he said. “The Syrian people have suffered for too long. The bloodshed must end now.”

While fighting rages in Damascus, clashes and shelling have also continued elsewhere across the country.

Rebels said they had “liberated” the town of Azaz in northern Aleppo province, bordering Turkey, and had taken over a border crossing at Bab al-Hawa gate, seizing control of the customs and immigration buildings on the Syrian side of the northern frontier.

Rebels also attacked Syrian forces at two spots along the nation’s porous border with Iraq, killing 21 soldiers and seizing control of one of the four major border posts, a senior Iraqi army official said.

“We have security concerns because the border crossing now is out of the Syria government’s control, and nobody can anticipate what will happen,” said Iraqi Army Brig. General Qassim al-Dulaimi.