BEIRUT — The United Nations estimated Wednesday that more than 60,000 people have been killed in Syria’s civil war, a toll one-third higher than what anti-regime activists had counted in the 21-month-old conflict. The U.N. human rights chief called the toll “truly shocking.”

Activists opposed to the regime of President Bashar Assad had been estimating the death toll at more than 45,000. This was the first time that the U.N. estimate surpassed those of activist groups.

“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,” U.N. 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.

“The failure of the international community, in particular the Security Council, to take concrete actions to stop the blood-letting, shames us all,” Pillay said. “Collectively, we have fiddled at the edges while Syria burns.”

An airstrike on a gas station in a Damascus suburb on Wednesday pushed that death toll even higher. Activists said a Syrian warplane blasted the station, killing and wounding dozens of people and igniting a huge fire in what may be one of the bloodiest attacks in weeks.

The new U.N. analysis also said monthly casualty figures have been steadily increasing since the conflict began in March 2011.