At least 25 people have been killed after an explosion rocked the skyscraper that houses the headquarters of Mexican oil giant Pemex in Mexico City, the country's interior minister has said.
"We have up to now 25 people dead - 17 women and eight men," Miguel Angel Osorio Chong told reporters, adding that 101 people were injured.
The state-owned 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.
"What took place was an explosion in the B2 building of the administrative centre. There are injuries and damage on the ground floor and mezzanine," Pemex said.
Dozens of employees were believed to be still trapped inside the building, more than 50 floors high, and rescue workers said the death toll could keep rising.
Emergency teams with rescue dogs, helicopters and several ambulances were at the scene.
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.
Television images showed heavy damage on the tower's ground floor and people being carted away on stretchers and office chairs.
"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.
Pemex, the world's fourth-largest producer of crude with around 2.5 million barrels per day, announced earlier that it had evacuated the building 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.