BEIRUT -- Activist groups said Sunday that about 5,000 people were killed in Syria's civil war in August, the highest figure ever reported in more than 17 months of fighting as President Bashar Assad's regime unleashed crushing air power against the revolt for the first time.
The U.N. children's fund UNICEF put the death toll for last week alone at 1,600, the largest weekly figure for the entire uprising.
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"The past month witnessed large massacres and the regime was conducting wide operations to try to crush the uprising," said Omar Idilbi, a Cairo-based activist with the Local Coordination Committees group. "Last month's acts of violence were unprecedented."
He said the increased use of the air force and artillery bombardments was behind the spike in casualties.
The civil war witnessed a major turning point in August when Assad's forces began widely using air power for the first time to try to put down the revolt. The fighting also reached Syria's largest city, Aleppo, which had been relatively quiet for most of the uprising.
The Britain-based activist group Syrian Observatory for Human Rights said that 5,440 people, including 4,114 civilians were killed in August. The LCC put the toll at 4,933 civilians.
On Sunday, the Observatory and the LCC said more than 100 people were killed throughout Syria and the groups have been reporting 100-250 deaths per day over the past week.
Syria's uprising has been the bloodiest in the Arab Spring that has already removed long-serving authoritarian leaders in Egypt, Tunisia, Yemen and Libya.
The two main activists groups also released new death tolls for the entire uprising since March 2011. The Observatory said more than 26,000 have been killed, including more than 18,500 civilians. The LCC put the death toll at more than 23,000 civilians. The LCC does not count members of the military who are killed, but the Observatory does.
That averages out to about 1,300-1,500 deaths per month, making the August figure more than three times higher than average.
The groups had previously reported a toll of around 20,000 more than a month ago.
On Thursday, Human Rights Watch said government forces have killed scores of civilians over the past three weeks by bombarding at least 10 areas where they were lining up to buy bread at bakeries near and around Aleppo.
Last week, activists reported that between 300 and 600 people were killed in the Damascus suburb of Daraya during days of shelling and a killing spree by troops who stormed the town after heavy fighting.
"The reason behind the high death toll is military operations, shelling, clashes and air raids," said Rami Abdul-Rahman who heads the Observatory.
"I would say most people are being killed during clashes and executions," he said referring to scores of bodies that are found in streets around Syria who are shot execution style with a bullet in the back of their heads.
UNICEF spokesman Patrick McCormick said 1,600 were killed last week alone, including some children. He did not immediately explain how he arrived at the figure, but said the number was documented.
As the death toll mounted, international efforts to end the crisis faltered badly. The U.N. and Arab League have both led prolonged but ultimately failed efforts to negotiate an end to the violence.
Turkey this week called for the U.N. to authorize creation of a safe zone in Syria for tens of thousands fleeing their homes. Britain and France have left open the possibility of more aggressive action, including a military-enforced no-fly zone to protect a safe area -- though that still seems a remote possibility.
Lakhdar Brahimi, the U.N.'s new envoy to Syria, told Assad's regime on Saturday that change is both "urgent" and "necessary" and that it must meet the "legitimate" demands of the Syrian people, words that will not win the seasoned Algerian diplomat and international trouble shooter any friends in Damascus.
On his first day on the job, Brahimi also called on both sides to end violence in Syria, but said Assad's government bears more responsibility than anyone else to halt the bloodshed.
While the military largely has been able to quell the offensive rebels launched in Damascus in July, it is still struggling to stamp out a rebel push in the northern city of Aleppo.
In the latest violence on Sunday, the Observatory said the military pounded rebel holdouts in Aleppo, the country's commercial capital. There was also fighting in other areas including the central city of Homs, Idlib province on the border of Turkey and suburbs near Damascus.
The Observatory said 21 people were killed when troops stormed the village of Alfan in the central province of Hama. It added that eight people were killed in the oil-rich eastern province of Deir el-Zour that borders Iraq.
An amateur video posted online showed more than a dozen bodies in Alfan covered with white shrouds in accordance with Islamic tradition as men and women sat around them crying and hugging the dead. A woman opened the shroud to see the face of one of the dead, then kissed it and started weeping.
Loud screams of prayers could be heard in a hall that appeared to be inside a mosque.
In the capital Damascus, two bombs exploded near the Syrian military's joint chiefs of staff offices, lightly wounding four army officers and damaging buildings and cars, state television reported. The twin blasts in the posh Abu Rummaneh district were the latest in a wave of bombings to hit Damascus in recent months as clashes between government troops and rebels reached the tightly controlled capital.
There was no immediate claim of responsibility for the bombings, which government officials said appeared to target a building under construction near the offices of the joint chiefs of staff. The building, which is officially known as the Guards Battalion and was empty at the time of the blast, serves as a base for army officers who guard the joint chiefs of staff offices some 200 meters (yards) away.
Several past bombings have targeted the security establishment in Damascus, most notably a July blast that killed four senior security officials, including the defense minister and his deputy, who was Assad's brother-in-law.
The government officials, speaking on condition of anonymity because they are not authorized to brief the media, said the wounded Sunday were army officers and they had minor injuries.
Footage broadcast on state TV showed a damaged building with debris strewn across the street. The blasts punched a hole in one of the building's walls, and blew out the windshield and windows of an SUV parked nearby.
The twin bombing was the second in recent weeks to hit Abu Rummaneh. On Aug. 15, a bomb attached to a fuel truck exploded outside the Dama Rose hotel where U.N. observers stayed before ending their mission to Syria. That blast, which hit a military compound parking lot, wounded three people.
Late Saturday, a car bomb near a Palestinian refugee camp in a suburb of Damascus killed at least 15 people, according to state news agency SANA. It said Sunday the explosion in the suburb of al-Sbeineh also wounded several people and caused heavy damage to buildings in the area.
It blamed the blast on an "armed terrorist group," the term the regime uses to describe the rebel Free Syrian Army seeking to topple Assad.
When Syria's unrest began, the country's half-million Palestinians at first struggled to remain on the sidelines. But in the past months, young Palestinian refugees -- enraged by mounting violence and moved by Arab Spring calls for greater freedoms -- have been taking to the streets and even joining the rebels.