Rescuers in Beirut are searching the rubble for a potential survivor after a possible heartbeat was detected 29 days after the massive explosion that flattened the city.

The remnants of the crushed building in the historic Mar Mikhael neighbourhood are being removed piece by piece which could take six hours.

A crowd has gathered at the scene, hoping for a miracle, as the Red Cross, fire crews and a rescue team from Chile lead the painstaking operation.

The discovery of possible life under the ruin was made when rescuers were walking past the site on Wednesday (local time) and their sniffer dog Fletch gave signs of something alive inside, the BBC reports.

When they returned the next morning, Fletch went to the same place and gave the same indications.

Specialist sensor equipment was brought in which detected a possible heartbeat and breath.

Fletch, a rescue dog of the Chilean rescue team, discovered possible signs of life on the rubble. Photo: Getty

The growing crowd has been urged to remain quiet to enable rescuers to listen for signs of life.

The discovery comes almost a month after the devastating Beirut explosion which killed nearly 200 people and left 300,000 homeless and led to the resignation of the Lebanese government.

The authorities said it was caused by about 2750 tonnes of ammonium nitrate that had been stacked in unsafe conditions in a port warehouse for years.

The blast smashed entire neighbourhoods, gutting buildings and injuring 6000 people.

Rescue workers dig through the rubble of a badly damaged building after detecting a heartbeat. Photo: Getty

Meanwhile the country’s army says it has found more explosive material, with 4.35 tonnes of ammonium nitrate found near the entrance to Beirut port, the site of the August 4 blast.

Army engineers were “dealing with it,” according to an army statement carried by the state news agency NNA.

The statement said the chemicals were found outside entrance nine to the port.

The explosion at the Port of Beirut on August 4 was felt across the city. Photo: Getty

The public remains anxious that more hazardous materials are being stored badly, putting them at risk.

Earlier on Thursday, President Michel Aoun ordered repairs to be made to old refuelling infrastructure at Beirut airport and called for an investigation into a report that thousands of litres of fuel had leaked from the system.

-with AAP