It was more than 72 hours after the quake and no cry for help could be heard from the rubble of Beichuan Middle School amid the din of heavy machinery around.

A rescue team from Chongqing was doing a last search on the school's collapsed main building, giving up hope of any survivors - when a sniffer dog barked.

Rescuers started moving heavy concrete chunks at the spot and two hours later, they found a girl in coma, her legs wounded badly, but still breathing.

She was reportedly the last survivor found in the school.

The efficacy and efficiency of sniffer dogs have become legion in the quake-hit areas of Sichuan, where they are often seen as the last line of hope.

More than 100 are working day and night, searching for signs of lingering life from corners and crevices that are beyond the reach of human senses.

"I've had my dog ever since he was a couple of months old but he's never been in such surroundings - the smell of bodies, the aftershocks and endless noise of power generators and hydraulic machines," said Qi Zhigang, keeper of Si Dao, a 6-year-old black Labrador.

Qi, a firefighter from Jiangsu, has been in Beichuan since last Thursday with a team accompanied by eight sniffer dogs.

"Si Dao has been searching on and on, and when he detects something he barks and scratches the rubble," Qi said. "He eats one meal per day and has slept only a couple of hours since we got here. He's really working hard."

Since Friday, Si Dao has managed to locate a few survivors but most of those he sniffed out were dead, Qi added.

As time runs out for survivors, rescue workers are pinning their hopes on dogs to guide men and machines to trapped people.

Sniffer dogs are deployed along the devastated main street of Yingxiu, Sichuan province, on Friday. More than 100 sniffer dogs are working day and night, searching for signs of lingering life in the quake-hit areas. [Photo: AP]

In Dujiangyan, a rescue team with seven dogs from Shandong province has soldiered on for almost 80 straight hours since last Tuesday and almost scoured the entire city.