* Interior minister says death toll could rise
* Cause of blast not clear
* At least 100 people injured (Adds details, background)
By Alexandra Alper
MEXICO CITY, Jan 31 (Reuters) - A powerful explosion rocked the Mexico City headquarters of state-owned oil giant Pemex on Thursday, killing at least 14 people, injuring 100 and trapping others inside.
The blast battered the lower floors of the downtown tower, throwing debris into the streets and sending frightened workers running outside.
It was not yet clear what caused the explosion, the latest in a series of safety problems to hit Mexico's national oil monopoly. Media reports said the incident occurred when machinery apparently exploded. An ambulance service official at the scene, who asked not to be named, said it was caused by a gas leak.
Interior Minister Miguel Angel Osorio Chong said the blast killed at least 14 people and injured 100. As evening fell, dozens of employees were believed to be still trapped inside, and rescue workers at the scene said the death toll at the Pemex skyscraper could keep rising.
Mauricio Parra, a paramedic at the scene, said he believed at least 20 had died and that 100 could be trapped inside.
Police quickly cordoned off the building, and television images showed the explosion caused serious damage to the ground floor and blew out windows on the lower floors of the tower.
"The place shook, we lost power and suddenly there was debris everywhere. Colleagues were helping us out of the building," witness Cristian Obele told Mexican television.
Some people at the scene said the blast came from a neighboring building, also part of the Pemex facilities.
Pemex said initially its headquarters had been evacuated because of a problem with its electricity supply. It then said there had been an explosion, but did not say what caused it.
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.
"Now we are in rescue mode and looking for people and for bodies," Osorio Chong said.
Search-and-rescue dogs were sent into the skyscraper, a Mexico City landmark more than 50 floors high and sporting a distinctive "hat" on top.
President Enrique Pena Nieto said via Twitter he "deeply regretted" the deaths and he headed to the scene of the blast.
Gloria Garcia, 53, a Pemex worker not in the building during the explosion, came to see if she could track down her son, who works in one of the floors hit.
"I'm calling his phone and he's not answering," Garcia said, weeping as she called repeatedly on her phone. "Nobody knows anything. They won't let me through. I want to see my son whatever state he's in."
Plaster fell from the ceiling of the basement, and the situation at the Pemex tower was dangerous, a spokesman for local emergency services said.
Pemex has experienced a number of deadly accidents in recent years and lesser safety problems have been a regular occurrence. In September, 30 people died after an explosion at a Pemex natural gas facility in northern Mexico.
More than 300 were killed when a Pemex natural gas plant on the outskirts of Mexico City exploded in 1984.
Eight years later, about 200 people were killed and 1,500 injured after a series of underground gas explosions in Guadalajara, Mexico's second biggest city. An official investigation found Pemex was partly to blame.
Alberto Islas, a security analyst at consultancy Risk Evaluation, said the explosion at the Pemex offices was another blot against the company's safety record.
"We've seen this time and again at Pemex. They don't have a well-integrated policy," Islas said, noting it would probably take several hours before investigators would be able to determine the cause of the explosion.
Pemex, a symbol of Mexican self-sufficiency since the oil industry was nationalized in 1938, has been held back by inefficiency and corruption and by the burden it shoulders of providing about a third of federal tax revenues.
Pena Nieto has pledged to open up the company to more private investment to improve its performance. (Additional reporting by Krista Hughes, Cyntia Barrera, Gabriel Stargardter and Liz Diaz; Writing by Dave Graham; Editing by Kieran Murray and Peter Cooney)