Rescue workers are digging through rubble in a wrecked district of Beirut in search of possible survivors from the massive port blast last month after scanners detected a pulse.

A sniffer dog - who was rescued himself from a shelter - responded to a scent from the site of a collapsed building.

The state news agency NNA reported the dog had detected movement under a destroyed building in the Gemmayze area, one of the worst hit by the blast that killed about 190 people and injured 6,000 others.

"These (signs of breathing and pulse) along with the temperature sensor means there is a possibility of life," rescue worker Eddy Bitar told reporters at the scene.

After several hours of digging through rubble, however, the operation was halted temporarily because the building was deemed too unsafe.

Heavier machinery was required to help lift the rubble safely, a rescue worker said, and it could not be brought until morning.

Fletch knows all about rescuing people - he is a rescue dog himself - and is pictured in Beirut today

The team of rescue workers included volunteers from Chile, as well as Lebanese volunteers and members of the civil defence.

News of the rescue prompted crowds to form at the site, who grew angry as rescue efforts were paused in a city desperate for hope.

Two hours later, there were cheers as members of the Lebanese civil defence returned to the scene and the rescue mission continued.

Beirut's governor Marwan Abboud told reporters: "There could be survivors. We hope someone will make it out alive."

Michel al-Mur of the Beirut fire department also said a pulse was detected about two metres below the rubble.

"One person, according to the (thermal) camera, still has a pulse," he said.

Speaking to local broadcaster LBCI, a rescue worker said scanners had picked up a respiratory rate of "19 breaths a minute".

Tomorrow, Lebanon marks one month since the blast that authorities say was caused by a shipment of ammonium nitrate left to languish for years at Beirut's port.