A huge blast rocked the headquarters of Mexican state-owned oil giant Pemex on Thursday, killing at least 25 people, injuring 100 and leaving rescuers scrambling to find survivors into the night.

Smoke billowed skyward as people fled the 54-floor Mexico City skyscraper, with some of those hurt in the blast being carried out on stretchers and office chairs, as witnesses recalled an earthquake-like rumble shaking the floor.

Windows broke on several lower floors, scattering debris. The company said the cause of the deadly incident was under investigation and declared that any reports on the origin of the blast amounted to speculation.

Officials said the blast ripped through an annex at around 3:40 pm (2140 GMT), causing severe damage to three floors. Witnesses said a roof connecting the annex to the tower collapsed. Thousands of people were evacuated.

As night fell, floodlights shined on the rubble and two cranes were brought to help rescuers find survivors.

Almost six hours after the blast, President Enrique Pena Nieto said on Twitter that "one more person was rescued alive in the rubble."

"I don't have any conclusive report on the cause, which is why I insist against any speculation," Pena Nieto told reporters after visiting the site.

A spokesman for the civil protection agency said there was an apparent "accumulation of gas" in an electrical supply room, but the exact cause of the blast has yet to be confirmed.

Interior Minister Miguel Angel Osorio Chong told a news conference that 17 women and eight men had died, and that 101 people had been injured.

Almost eight hours after the blast, he said it was hard to know if anybody was still trapped in the rubble.

Pena Nieto ordered that the rescue continue "to the last piece of debris, to be sure that there is not one single person trapped under," the minister said.

Emergency workers with rescue dogs, helicopters and several ambulances were at the scene in the capital, a city that is equipped to handle earthquakes.

Pemex said on Twitter that the explosion hit the ground floor and mezzanine of the tower's annex.

"We had two minutes to leave the building. I was headed to the pharmacy when the windows broke. It was a deafening noise," Astrid Garcia Trevino, who worked in the annex, told AFP. "The floor shook as if it was an earthquake."

Some witnesses told local media that a number of people were trapped in rubble.

"It was dramatic. The building was shaking and suddenly there was debris. We couldn't even see the people next to us," Pemex employee and union member Cristian Obele told reporters.

"Windows broke, people were injured and a lot of people were in shock," an unidentified worker told the Televisa channel, describing the impact of the "very strong explosion."

Mayor Miguel Angel Mancera and several top federal officials headed to the scene, as authorities closed off the avenue in front of the tower.

"I deeply regret the death of Pemex workers," Pena Nieto said on Twitter. "The priority at the moment is to help the injured and protect the people working there."

Pemex, the world's fourth-largest crude producer with around 2.5 million barrels per day, said the administrative center would remain closed "until further notice," after it had earlier been evacuated due to a power failure.

The company has experienced deadly accidents at its oil and gas facilities in the past. Last year, a huge explosion killed 30 people at a gas plant near the northern city of Reynosa, close to the US border.

The previous worst incident took place in December 2010, when an oil pipeline exploded after it was punctured by thieves in the central town of San Martin Texmelucan, leaving 29 dead and injuring more than 50.

In October 2007, 21 Pemex workers died during a gas leak on an oil platform in the Gulf of Mexico. Most drowned when they jumped into the sea in panic.