Paramedics wheel an injured person to a helicopter at the parking lot of the state-run oil company Pemex after an explosion in Mexico City Jan 31, 2013.[Photo/Agencies]

MEXICO CITY - A powerful explosion rocked the Mexico City headquarters of state-owned oil giant Pemex on Thursday, killing at least 25 people, injuring more than 100 and trapping others inside.

The mid-afternoon blast shattered the lower floors of the downtown tower, throwing debris into the streets and sending frightened workers running outside.

A government official, speaking on condition of anonymity, said preliminary findings suggested the blast was caused by a gas boiler exploding in a Pemex building next to the tower. But the cause was still being investigated, the official added.

The explosion was the latest in a series of safety problems to hit Mexico's national oil monopoly.

Interior Minister Miguel Angel Osorio Chong said the blast killed at least 25 people, up from a previous count of 14, and injured 100. Dozens of employees were believed to be still trapped inside, and rescue workers said the death toll at the Pemex skyscraper could keep rising.

Mauricio Parra, a paramedic at the scene, said he believed at least 20 people had died and that 100 could be trapped at the offices of Pemex, a national institution that President Enrique Pena Nieto's administration has pledged to reform this year.

Police quickly cordoned off the building, and television images showed the explosion caused major damage to the ground floor and blew out windows on the lower floors of the tower.

"You could feel it all through the building," said Mario Guzman, a Pemex worker who was on the 10th floor of the building, which is more than 50 floors high.

First mistaking the blast for an earthquake, Guzman, who said he escaped after running down the stairs, feared the building would collapse on top of him and his colleagues,   "and that we would end up like a sandwich."

Several witnesses said the blast came from the neighboring Pemex building. Pemex said initially the tower was evacuated due to a problem with its electricity supply. It then said there had been an explosion, but did not say what caused it.

Earlier in the evening, Pena Nieto, who took office in December, went to the scene of the blast and said it would be thoroughly investigated. He vowed to apply "the force of the law" if anyone was found to be responsible for it.

Mexican news network Milenio said security officials after the explosion carried out a precautionary search of Congress for explosive devices, but found nothing.

Helicopters buzzed around the building and lines of fire trucks sped to the entrance, while emergency workers ferried injured people through wreckage strewn on the street.

Search-and-rescue dogs were sent into the skyscraper, a Mexico City landmark that sports a distinctive "hat" on top. Pemex published a list of 105 workers who were being treated for injuries in hospitals.