WASHINGTON, May 18 — Casualties among private contractors in Iraq have soared to record levels this year, setting a pace that seems certain to turn 2007 into the bloodiest year yet for the civilians who work alongside the American military in the war zone, according to new government numbers.

At least 146 contract workers were killed in Iraq in the first three months of the year, by far the highest number for any quarter since the war began in March 2003, according to the Labor Department, which processes death and injury claims for those working as United States government contractors in Iraq.

That brings the total number of contractors killed in Iraq to at least 917, along with more than 12,000 wounded in battle or injured on the job, according to government figures and dozens of interviews.

The numbers, which have not been previously reported, disclose the extent to which contractors — Americans, Iraqis and workers from more than three dozen other countries — are largely hidden casualties of the war, and now are facing increased risks alongside American soldiers and marines as President Bush’s plan to increase troop levels in Baghdad takes hold.