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 UN human rights chief called the toll "truly shocking."

Activists opposed to the regime of President Bashar al-Assad had been estimating the death toll at more than 45,000. This was the first time that the UN 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," 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.

"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."

Death toll likely greater

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 UN analysis also said monthly casualty figures have been steadily increasing since the conflict began in March 2011.

This citizen journalism image taken from video provided by Shaam News Network shows a wounded man being pulled from the site of a Syrian government airstrike on a gas station in the eastern Damascus suburb of Mleiha on Wednesday. (Shaam News Network/Associated Press Video)

Independent experts compared 147,349 killings reported by seven different sources — including the government — for the study, which was commissioned by the UN human rights office.

By removing duplicates they arrived at a list of 59,648 individuals killed between the start of the uprising on March 15, 2011, and Nov. 30, 2012. In each case, the victim's first and last name, the date and the location of his or her death were known.

The real death toll is likely to be even greater because reports containing incomplete information were excluded and a significant number of killings may not have been documented at all by the sources available.

"There are many names not on the list for people who were quietly shot in the woods," Pillay's spokesman Rupert Colville told The Associated Press.

The data, which didn't distinguish among soldiers, rebels and civilians, also show that the killing in Syria has accelerated.

During the summer of 2011, shortly after the uprising against Assad began, the monthly death toll stood at around 1,000. A year later, an average of 5,000 were killed each month, the UN said.

Most of the killings occurred in Homs, followed by rural Damascus, Idlib, Aleppo, Daraa and Hama. At least three quarters of the victims were male.

Missile struck as drivers waited at station for gas

The UN rights chief warned that thousands more would die or suffer terrible injuries if the conflict continues, and repeated her call that those responsible for the killings — which in some cases could amount to war crimes — should be held accountable.

"We must not compound the existing disaster by failing to prepare for the inevitable — and very dangerous — instability that will occur when the conflict ends," she said.

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"Serious planning needs to get under way immediately, not just to provide humanitarian aid to all those who need it, but to protect all Syrian citizens from extra-judicial reprisals and acts of revenge" like those seen in Afghanistan, Iraq, Somalia and Congo, she said.

What began with peaceful protests has evolved into a full-scale civil war in Syria, with scores of armed groups fighting regime forces across the Arab country.

Anti-regime activists said Wednesday that a single Russian-built MiG fighter fired a missile that hit the gas station, setting off an inferno in the eastern suburb of Mleiha. Black smoke billowed from the site. An amateur video posted online showed charred bodies and gruesome carnage at the scene.

Mohammed Saeed, an activist who visited the site, said the missile struck as drivers waited in line with their cars at the station. Syria has been facing a fuel crisis, and people often must wait for hours to get gasoline.

Meanwhile, rebels have been targeting airports, including the Mannagh military helicopter base near the Turkish border.