UNITED NATIONS - More than 60,000 people have been killed in Syria’s 21-month-old uprising against President Bashar al-Assad’s government, the UN human rights commissioner said Wednesday, citing an “exhaustive” study which has sharply increased the number of those believed killed.

Opposition activist groups had been estimating the death toll at more than 45,000 and this was the first time that the UN estimate was higher.

“Given there has been no letup in the conflict since the end of November, we can assume that more than 60,000 people have been killed by the beginning of 2013,” UN High Commissioner for Human Rights Navi Pillay said in a statement. “The number of casualties is much higher than we expected, and is truly shocking,” she added.

According to a news release issued by the Geneva-based Office of UN High Commissioner for Human Rights, the preliminary analysis, which took five months to complete, was conducted using a combined list of 147,349 reported killings, fully identified by the first and last name of the victims, as well as the date and location of the deaths.

Any reported killing that did not include at least these four elements was excluded from the list, which was compiled using datasets from seven different sources, including the Syrian Government.

The analysts noted that 60,000 is likely to be an underestimate of the actual number of deaths, given that reports containing insufficient information were excluded from the list, and that a significant number of killings may not have been documented at all by any of the seven sources.

“Although this is the most detailed and wide-ranging analysis of casualty figures so far, this is by no means a definitive figure,” Pillay noted. “We have not been able to verify the circumstances of each and every death, partly because of the nature of the conflict and partly because we have not been allowed inside Syria since the unrest began in March 2011.

AFP add: The casualties continued to pile up on Wednesday, when a regime air strike in the Eastern Ghuta region of Damascus killed or wounded dozens of people, many of them horribly burned, the Observatory said.

“There are 12 bodies that have been found at the scene, but it is not yet clear whether they were civilians or rebels or if the gas station was the target,” Observatory director Rami Abdel Rahman told AFP by phone. The Local Coordination Committees, a grassroots network of activists, estimated that at least 50 people were killed and dozens of others wounded.

It said the toll was likely to rise because bodies were still being pulled from the rubble, adding that “it is extremely difficult to count the dead because most of the bodies have been immolated.”

“MIG warplane strikes on Eastern Ghuta! Dozens of martyrs!” a man in the video shouted out as he and a fellow cameraman raced toward plumes of smoke to survey the damage.

Elsewhere, the Britain-based Observatory said regime warplanes bombed the town of Moadamiyet al-Sham, southwest of the capital, killing 12 members of the same family, most of them children. The strikes came as President Bashar al-Assad’s forces shelled rebels in insurgent strongholds to the northeast of the capital, and in Daraya to the southwest. South of the capital, residents of a Palestinian refugee camp that faced air raids in December were sent fleeing once again by barrages of mortar fire.

In northwestern Syria, clashes between mostly rebel fighters and Assad’s forces at Taftanaz airbase in Idlib province killed four insurgents and an unknown number of soldiers, the Observatory said. Fighting also broke out around the crucial Wadi Deif base, one of the last regime bastions in the northwest, in a fresh bid to wrest control of the strategic post.

 A freelance American reporter who contributed videos to Agence France-Presse (AFP) in recent months was kidnapped in war-torn Syria six weeks ago and has been missing since, his family revealed Wednesday. US journalist James Foley, 39, an experienced war reporter who has covered other conflicts, was seized by armed men in the northern province of Idlib on November 22, according to witnesses.

Foley, who last supplied AFP with video material the day before he disappeared, was picked up in the town of Taftanaz by four men who later released his driver and translator, the witnesses said.

No group has claimed responsibility for the abduction, in a country which has been racked by bitter civil war since the start of an uprising by rebels opposing President Bashar al-Assad’s regime nearly two years ago.