Update: The death toll from the the blast that collapsed the lower floors of Pemex headquarters in Mexico City is now up to at least 33 people. Director-General Emilio Lozoya of Mexico's state-owned oil company said in a statement that the blast, which produced tons of rubble and injured an additional 121 people, is being looked at as an accident - although all lines of investigation remain open. Early signs point to a problem with electrical and air-conditioning equipment, according to a government official who was not authorized to speak.

More than 500 firefighters, soldiers, and rescue workers are digging through chunks of concrete as they continue to look for additional victims or survivors. Fifty-two survivors the are still hospitalized.

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Mexico City, Mexico - At least 14 were killed and more than 100 injured on Thursday, after an explosion rocked the skyscraper headquarters of Pemex, Mexico's state oil company. A further 30 people were said to be trapped in the base of the tower block following the blast, which caused fire and rising smoke as ambulances flooded to the scene.

Mexican authorities could not say what caused the blast but the company had reported having electrical problems in the building earlier in the day.

"We have 13 dead at the scene and one more at the hospital. There are more than 80 wounded and we continue to look for survivors in the debris," said Miguel Angel Osorio Chong, the interior minister. Footage shot from a helicopter circling the scene showed the injured being taken out on stretcher and in some cases wheeled away in office chairs.

"What took place was an explosion in the B2 building of the administrative center. There are injuries and damage on the ground floor and mezzanine," Pemex said in a statement.

Earlier in the afternoon the company said that its headquarters was suffering "a fault in the power supply" and was being evacuated as a precaution. One ambulance official told reporters that a gas leak in the building appeared to be the source of the explosion.

"The place shook, we lost power, and suddenly there was debris everywhere. Colleagues were helping us out of the building," one witness told a Mexican television station reporter.

Television pictures showed rubble from the blast spread out on to the street in front of the building, and Red Cross ambulances on the scene attending to the injured. Emergency services, assisted by dogs, were seen searching for people trapped in the rubble.

The explosion, which occurred around 3:45 pm, caused chaos in the streets and snarling traffic leading to road closures around the building, which is located in a very busy commercial area of Mexico City. Relatives of employees are gathered outside the building in search of information about loved ones.

The firm's chief executive, Emilio Lozoya Austin cut short a business trip to Asia and was making his way back to Mexico.

President Enrique Pena Nieto said via his Twitter account, "I profoundly lament the death of our fellow workers at Pemex. My condolences to their families. At the moment, the priority is to help the injured and protect the physical safety of those who work there."

The president said he has ordered an investigation into the causes of the blast.

Pemex is the world's 4th largest crude oil producer, pumping out around 2.5 million barrels per day.