The data did not distinguish among soldiers, rebels or civilians.
It indicated that the pace of killing has accelerated. Monthly death tolls in summer 2011 were around 1,000. A year later, they had reached about 5,000 per month.
Most of the killings were in the province of Homs, followed by the Damascus suburbs, Idlib, Aleppo, Daraa and Hama. At least three-fourths of the victims were male.
Pillay warned that thousands more could die or be injured, and she said the danger could continue even after the war.
"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.
The U.N. refugee agency said about 84,000 people fled Syria in December alone, bringing the total number of refugees to about a half-million. Many more are displaced inside Syria.
While no one expects the war to end soon, international sanctions and rebel advances are eroding Assad's power. Rebels recently have targeted two pillars of his strength: his control of the skies and his grip on Damascus.
Rebels in northern Syria attacked a government helicopter base near the village of Taftanaz in Idlib province, activists said. Videos posted online showed them blasting targets inside the airport with heavy machine guns mounted on trucks.
All videos appeared genuine and corresponded with other AP reporting on the events.
In recent weeks, rebels have attacked three other airports in north Syria. They clashed Wednesday with forces inside the Mannagh military airport near the Turkish border as well as near the Aleppo international airport and adjacent Nerab military airport, halting air traffic there for the second straight day.
The fall of those airports to the rebels would embarrass the regime but not fully stop the airstrikes by government jets, many of which come from bases farther south.
In another blow to the regime and to Syria's economy, a company based in the Philippines that handled shipping containers at Syria's largest port said it was canceling its contract, citing an "untenable, hostile and dangerous business environment."
The Manila-based International Container Terminal Services Inc. said the amount of port traffic had gone down, hurting business, while conditions in Syria grew more dangerous.
The company's departure will significantly limit cargo services at the Tartus port.
Also, Wednesday, the family of American journalist James Foley revealed that he has been missing in Syria for more than a month. Foley was providing video for Agence France-Press when he was abducted Nov. 22 by unknown gunmen, his family said in a statement.
"His captors, whoever they may be, must release him immediately," said AFP chairman Emmanuel Hoog.
Covering Syria has been a challenge for journalists. The government rarely gives visas to journalists, prompting some to sneak in with the rebels, often at great danger.
Jordans reported from Berlin. Associated Press writers Bassem Mroue in Beirut, Elaine Ganley in Paris, Jim Gomez in Manila, Philippines, and Juergen Baetz in Geneva contributed reporting.