Television footage of the complex, including its 52-story tower, showed people being carried away on stretchers and office chairs. Pemex announced that the company itself had evacuated the building where several thousand people work.

A blast heavily damaged the tower's lower main floor and the mezzanine of an auxiliary building the previous day. Windows as far as three floors up were blown out.

As rescue workers continued their search Thursday morning, it was unclear how many of the company's employees were still trapped inside.

The death toll rose sharply to over 30 people by midmorning, according to the head of Pemex, Emilio Lozoya. Original numbers by the government had counted 15 casualties.

Mexico's Attorney General's Office said it was analyzing the explosion and reports about likely causes were only speculative. Pemex had initially cited an electrical supply problem. Company sources said air-conditioning was overheated.

A spokesman for the civil protection agency initially told the news agency AFP that the blast was caused by "an accumulation of gas" in an electrical supply room on a lower floor.

"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris," employee Cristian Obele told Milenio television.

"We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows," said Maria Concepcion Andrade, 42, who lives close to the Pemex building.

"People started running from the building covered in dust," she said. "A lot of pieces were flying."

Police landed four rescue helicopters to remove the dead or injured. About a dozen tow trucks hurriedly moved cars to make more landing room for the helicopters.

Streets surrounding the building were closed as evacuees wandered around and rescue crews loaded the injured into ambulances.

Pemex, which is the world's fourth-largest producer of crude oil, has a record of deadly accidents.

Last year, 30 people were killed by an explosion at a gas plant in Reynosa, near Mexico's border with the US. In December 2010, an oil pipeline punctured by thieves exploded in central Mexico, killing 29 people.

ipj/slk (AP, AFP, Reuters, dpa)