Iranian rescue workers are digging tunnels to reach the underground stories of the ageing high-rise that collapsed in downtown Tehran amid ongoing search operations to retrieve those trapped.

Plasco Building caught fire in early Thursday and came down only hours later, blockading 25 to 30 people under the detritus.

Rescuers using listening equipment in the early hours of Saturday morning detected responses from four locations under the rubble, meaning at least four people could still be alive, the Mehr news agency reported.

Meanwhile, Tehran Fire Department spokesman Jalal Maleki has confirmed the death of at least 16 firefighters, putting the total number of the bodies trapped under the rubble, including those of the firefighters, at 25.

He told state TV on Friday night that four bodies had so far been retrieved from the rubble.

The developments came amid reports that rescue workers had to conduct a controlled demolition of an adjacent two-storey building that had caught fire in a bid to reach those trapped under the rubble.

Earlier, Saeid Sharif-Zadeh, the chief executive of Iran’s Fire Control Organization, said two tunnels were dug into the first and second stories beneath the building’s floor.

Rescuers, he said, were, however, yet to reach those trapped there. The relief workers were deploying technical equipment and sniffer dogs to locate potential survivors.

Rescue operations continue at the site of a building collapse in central Tehran, Iran, January 20, 2017. (Photo by Mehr)

Barely-above-0° temperatures stand to complicate the operation.

Late Thursday, Tehran Mayor Mohammad-Baqer Qalibaf said as many as 20 firefighters had died during the disaster.

On Friday morning, however, IRNA quoted a familiar source as saying that as many as 23 firefighters and some four journalists remain buried under the rubble.

But Tehran Police Chief Brigadier General Hossein Sajedi-Nia said later that only two people other than 20 firefighters were still unaccounted for after the collapse of the building.

Rescue operations continue at the site of a building collapse in central Tehran, Iran, January 20, 2017. (Photo by Mehr)

Pir-Hossein Koulivand, the head of the capital’s Emergency Services, later announced that one firefighter, who had been rushed to hospital for treatment, had died of his injuries after several resuscitation efforts failed to sustain him.

Koulivand also said 84 people had suffered injuries during the collapse, 33 of whom were still hospitalized.

“The relief work is very hard. Removal of debris is going on in all parts of the building but the thick smoke rising from the building is creating serious problems,” he said.

The photo shows the aftermath of the collapse of an ageing high-rise earlier hit by fire in Tehran on January 19, 2017. (By IRNA)

He said “it is still not clear how many people are trapped under the rubble and not even one person has been pulled out.”

Esmail Najjar, the head of Tehran’s crisis management center, also told ISNA that it was “very unlikely that we will pull anyone out alive from the rubble.”

Later on Friday, the Iranian administration declared Saturday a day of national mourning for the victims, and extended condolences to their families and hailed rescue teams for their bravery and sacrifice.

Iranian Interior Minister Abdolreza Rahmani Fazli inspected the site of the fire and collapse early Friday.

A firefighter consoles his comrade after the collapse of Iran’s oldest high-rise in downtown Tehran on January 19, 2017. (Photo by AFP)

He has tasked police forces with preventing citizens from crowding the area as rescue teams operating there are on high alert for possible blasts in the aftermath of the collapse.

As many as 200 firefighters had been deployed to the site to put out the fire.

Plasco Building, Iran’s oldest high-rise, stands in downtown Tehran prior to its collapse after being hit by a massive fire.

The building was one of the capital’s oldest high-rises, which was housing 600 production workshops, 400 of which were being used for making shirts.

Material damage from the collapse is estimated to stand at around 15,000 billion rials ($463 million).

Leader of Islamic Revolution Ayatollah Seyyed Ali Khamenei has issued a message, addressing the situation. Iran’s President has ordered expedient handling of the disaster, while First Vice President Is’haq Jahangiri has attended the scene.

Foreign governments, including those of Syria, Pakistan, Turkey and Britain, have condoled with the Islamic Republic over the incident.