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.

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 cleanup until they were certain no survivors or even bodies remained.

The 72-hour-mark expired at 1:14 p.m. (1814 GMT) on Friday.

Three days is the limit that experts say people trapped in rubble without water, often with crushed limbs, can hold on.

Families clung to hope as they watched rescue teams painstakingly work through the jumbled wreckage.

But psychologists dispatched to the scene were already preparing to help relatives deal with tragic news.

“The families are still hopeful, but we psychologists are starting to prepare ourselves to counsel them in the context of mourning,” said Penelope Exzacarias, who was on standby at a collapsed office building in Mexico City’s trendy Roma neighborhood.

Some 70 people were at work in the building when the 7.1-magnitude earthquake struck. Only 28 have made it out alive so far — all in the first hours.

But Mexicans remember “miracle” rescues a week after a 1985 quake that killed more than 10,000 people in the capital.

The government’s open-ended extension on rescue efforts posed a dilemma for emergency workers in the ruins of a clothing factory, just one of nearly 40 collapsed buildings in the capital.

Continue, but how long?

“There are no indications of anyone inside but they’re not sure enough to affirm there’s really no one. The camera used doesn’t allow a full view,” explained Daniel Quiroz, a 22-year-old volunteer.

So far, 115 people have been pulled alive from the rubble, according to the military.

In all likelihood, the death toll will rise above the latest figure of 295.

Mexico City recorded the highest number of fatalities: 157, with more bodies certain to be found.

The rest of the deaths occurred in the states of Morelos, Mexico, Puebla, Guerrero and Oaxaca.

The toll includes eight foreigners: four Taiwanese, a Korean, a Spaniard, a Panamanian and an Argentine, authorities said.

Several countries, including the United States, Israel, Panama and EU states have sent crews to help the rescue effort.

A Japanese team was using a hi-tech scanner on the toppled building in the Roma district.

Jose Gutierrez, father of one of those trapped, and also a civil engineer, warned that “the structure is at risk of total collapse.”

Overnight rain caused saturated debris to become heavier and shift, he said.

In the south of Mexico City, at a flattened school where 19 children died, white wreaths lay out, testimony to the mourning of relatives and neighbors.

Families were starting to hold funerals. — AFP