An office building blast kills 25 people and injures 101 at the headquarters of Mexico's state-owned oil company, Petroleos Mexicanos.

Interior Minister Miguel Osorio Chong visits the scene of the Mexico City blast (Reuters)

Rescuers searched the rubble for survivors and authorities promised a thorough investigation following the explosion in the basement of an administrative building next to the iconic, 51-storey Pemex tower in Mexico City.

The cause of the blast on Thursday afternoon remained a mystery on Friday morning, with the country's President Enrique Pena Nieto urging people not to speculate. Theories ranged from an electrical fire to an air conditioning problem to a possible attack.

"We have no conclusive report on the reason," Mr Pena Nieto told reporters, promising to find out what happened and to hold anyone responsible to account.

Some 46 people remained hospitalized after the blast, some gravely injured and others with cuts, fractures and burns. Authorities said the dead were 17 women and eight men.

Search continues

More than 500 firefighters, soldiers and rescue workers dug through chunks of concrete with dogs, trucks and a Pemex crane.

Interior Minister Miguel Osorio Chong said it was uncertain if any of the roughly 10,000 people who work in the five-building headquarters were still trapped, but that the search would continue.

The explosion occurred at about 15:45 local time, just as the administrative shift was about to end. It hit the basement and first two floors, which rescuers said all collapsed onto each other.

Gabriela Espinoza, 50, a Pemex secretary for 29 years, was on the second floor of the tower when she said she heard two loud explosions and a third smaller one.

"There was a very loud roar. It was very ugly," she said.

Espinoza's colleague, Tomas Rivera, 32, worked on the ground floor where the explosion occurred and said the force knocked him to the basement, fracturing his wrist and jaw. The injured were taken to two nearby hospitals and other medical facilities, where relatives huddled in the waiting room for news of their loved ones.

Some walked out of meetings with the hospital social worker joyous, while others came out crying.

An elderly man is helped outside the Pemex building (Reuters)

White smoke

"We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows," said Maria Concepcion Andrade, who lives on the same block as the Pemex building. "People started running from the building covered in dust. A lot of pieces were flying."

Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia Garcia, 36, who works in the building where the explosion occurred. She said she last talked to him a couple hours earlier.

"I called his phone many times, but a young man answered and told me he found the phone in the debris," Vargas said.

Pemex, created as a state-owned company in 1938, has nearly 150,000 employees and in 2011 produced about 2.5 million barrels of crude oil a day, according to its website, with $111bn in sales.

Pena Nieto, who took office in December, has made Pemex reform the centre of his platform, with a plan to pump new investment into a company whose profits feed much of Mexico's federal budget, but which has fallen behind other oil companies in production, technology and exploration.

Shortly before the explosion, the company's operations director Carlos Murrieta reported via Twitter that the company had reduced its accident rate in recent years. Most Pemex accidents have occurred at pipeline and refinery installations.

A fire at a pipeline metering centre in northeast Mexico near the Texas border killed 30 workers in September, the largest-single toll in at least a decade for the company.

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