"We now face the reality that there is no chance that anyone could have survived this long," Civil Defense Emergency Management national controller John Hamilton told a news conference Thursday.
The families of some 200 missing people are devastated.
Rescuers have pulled 161 bodies from the rubble. Most have not yet been identified. Dozens more dead are thought to still be trapped, leaving hundreds of families in anguish.
Officials say the death toll could eventually rise to as high as 240 dead.
More than 900 workers rushed to Christchurch from several countries after the Feb. 22 earthquake and have been picking through the wreckage.
Rescuers saved 70 people in the first 26 hours after the quake struck just before 1 p.m. on Feb. 22, but no one has been found alive since.
The Associated Press contributed to this story.