Rescue workers digging through the rubble of a Beirut building for the third day have said there is no longer hope of finding someone alive more than a month after a huge port explosion shattered Lebanon's capital.

About 50 rescue workers and volunteers, including a specialist team from Chile, had been working for three days to locate any survivors after sensors on Thursday detected signs of breathing and heat.

"Technically speaking, there are no signs of life inside the building," Francisco Lermanda, the head of volunteer rescue group Topos Chile, said in a news conference on Saturday evening, adding that rescuers had combed 95 percent of the building.

The signs of life picked up by their sensitive equipment in the past two days, Lermanda said, were the breaths of fellow rescuers who were already inside the building. He said efforts would now focus on clearing the rubble and finding remains.

"We never stop with even 1 percent of hope," Lermanda said. "We never stop until the job is done."

The rescue efforts had dominated local and social media for days, as the Lebanese were transfixed, desperate for a miracle. But none came.

But even the faint hope of a miracle caught the imagination of a country already reeling from the coronavirus pandemic and the country's worst economic crisis in decades.

"I was not aware I needed a miracle that much. Please God, give Beirut this miracle it deserves," said Selim Mourad, a 32-year-old filmmaker.

'Crumbling' building

The August 4 blast killed some 200 people, injured 6,000 more, and devastated whole neighbourhoods. One month on, seven people are still listed as missing.

The authorities held ceremonies on Friday to mark a month since the explosion tore into the city.

The ruined building where the search was continuing lies between the residential districts of Gemmayze and Mar Mikhael, among the hardest-hit areas by the blast and home to many old buildings that crumbled as the shockwave ripped through.

Work was slow, rescue workers said, as the badly damaged building was at risk of complete collapse.

"The building is really crumbling, it's scary and there's a lot of danger to the team," said George Abou Moussa, head of Lebanon's civil defence, on Saturday evening.

Moussa in the morning had said the chances of finding someone alive were "very low".

Workers used shovels and their hands to dig, while mechanical diggers and a crane lifted heavy debris. Scanning equipment was also used to create 3D images of the wrecked building.