She said Assad "has used indiscriminate and disproportionate force against Syrian civilians and inflicted unthinkable suffering upon his own people."
The Britain-based Syrian Observatory for Human Rights, which has its own count based on a network of activists around the country, said 98,416 people have been killed from the start of the uprising until Wednesday, many of them members of the military or pro-regime militiamen.
The Observatory's figures include 36,139 civilians and 42,147 regime fighters, including military and defense forces, as well as pro-government militias known as the Popular Committees, shabiha and National Defense Forces.
"The real figure could be higher than 120,000 or 130,000," said Rami Abdul-Rahman, the head of the Observatory, citing regime secrecy on its losses and the difficulties for human rights group to work inside Syria.
The soaring death toll underscores the brutal nature of the conflict in Syria, even in comparison with sectarian conflicts that ravaged neighbors Lebanon and Iraq.
A review by the Associated Press in April 2009 showed that more than 110,600 Iraqis died in violence since the U.S.-led invasion six years earlier. The actual number was likely higher because many of those listed as missing were doubtless buried in the chaos of war without official records.
About 150,000 people are believed to have died in Lebanon's 15-year civil war, which ended in 1990, and some 17,000 people are still missing. Sectarian sensitivities still prevail in Lebanon, more than two decades after the war ended.
The U.N. figures released Thursday in Geneva trace the arc of violence, with the average monthly number of documented killings rising from around 1,000 per month in summer 2011 to an average of more than 5,000 per month since July 2012. At its height from July to October 2012, the number of killings rose above 6,000 per month.
"The constant flow of killings continues at shockingly high levels," Pillay said. "This is most likely a minimum casualty figure. The true number of those killed is potentially much higher."
Among the victims were at least 6,561 children, including 1,729 younger than 10, the U.N. said.
"There are also well-documented cases of individual children being tortured and executed, and entire families including babies being massacred — which, along with this devastatingly high death toll, is a terrible reminder of just how vicious this conflict has become," Pillay said.
"Civilians are bearing the brunt of widespread, violent and often indiscriminate attacks, which are devastating whole swaths of major towns and cities, as well as outlying villages," Pillay said.
"Government forces are shelling and launching aerial attacks on urban areas day in and day out," she said. "Opposition forces have also shelled residential areas, albeit using less firepower, and there have been multiple bombings resulting in casualties in the heart of cities, especially Damascus."
Syrian Transportation Minister Mahmoud Ibrahim Said told Syrian TV that a mortar round fired by "terrorists" struck near a warehouse, breaking its windows and wounding a worker.
He said the attack delayed the landing of two incoming flights, from Latakia and Kuwait, as well as a flight to Baghdad. No passengers were harmed and no planes were damaged, he said. The regime commonly refers to rebels as "terrorists."
Tarek Wahibi, head of operations at the airport, said arrivals and departures later resumed to normal.
Rebels also battled regime forces for control of a key military base in the central province of Hama after chasing soldiers out and setting fire to installations there, activists said.
Following dawn battles, rebels took control of the base on the northern edge of the town of Morek, which straddles the strategic north-south highway leading to Aleppo.