MEXICO CITY: Mexican rescuers were working through the early hours Saturday in a desperate search for survivors of an earthquake that killed nearly 300 people, hoping to defy experts who say the chances of finding life in the rubble after 72 hours are bleak, reports AFP.

With exhausted emergency workers still reporting signs of life at several spots in Mexico City, the head of the national disaster management agency, Luis Felipe Puente, warned the coming hours would be critical.

"Tonight will be tough, because a lot of time has passed (since Tuesday's quake). But we won't give up," he said.

"Time has gotten the best of us. There are structures that are very complicated to access. But we're going to keep fighting for the families" of those feared trapped inside, he told TV network Televisa.

He said the authorities would not bring in the bulldozers to start clean-up until they were certain no survivors or even bodies remained. The 72-hour-mark expired at 1:14 pm (1814 GMT) Friday. Three days is the limit that experts say people trapped in rubble without water, often with crushed limbs, can hold on.