BAGHDAD, Iraq — The United Nations said today that more than 34,000 Iraqi civilians were killed in sectarian violence last year, nearly three times the number reported dead by the Iraqi government.

The Iraqi Health Ministry did not comment on the report, which was based on information released by the Iraqi government and hospitals. The government has disputed previous figures released by the U.N. as "inaccurate and exaggerated."

The same day the figure was released, two back-to-back explosions struck a used motorcycle marketplace in central Baghdad, killing at least 15 people and wounding 74.

The first bomb was attached to a motorcycle in the market. As the curious gathered to look at the aftermath, a suicide car bomber drove into the crowd and blew up his vehicle. Authorities said at least three policemen were among the dead.

The blast appeared to target the mainly Shiite neighborhood near the market but also was near the Sheik al-Gailani shrine, one of the holiest Sunni locations in the capital.

The U.N. figure was released as Baghdad braces for a major security operation to be launched by the Iraqi government and U.S. forces aimed at quelling the rampant sectarian violence that has been on the rise since the Feb. 22 bombing of a Shiite mosque in Samarra.

Much of the violence has been blamed on Shiite militias, particularly the Mahdi Army militia. Dozens of bodies turn up on the streets of Baghdad daily, many showing signs of torture.

Gianni Magazzeni, the chief of the U.N. Assistance Mission for Iraq in Baghdad, said 34,452 civilians were killed and 36,685 were wounded last year.

Iraqi government figures announced in early January put last year's civilian death toll at 12,357. When asked about the difference, Magazzeni said the U.N. figures were compiled from information obtained through the Iraqi Health Ministry, hospitals across the country and the Medico-Legal Institute in Baghdad.

"Without significant progress in the rule of law sectarian violence will continue indefinitely and eventually spiral out of control," he warned.

Magazzeni said 6,376 civilians were killed violently in November and December — 4,731 of those in Baghdad, most as a result of gunshot wounds.