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.

Tackling the poverty that blights Mexico has been a priority for many presidents of Latin America’s second-biggest economy, and it was one of the first pledges Pena Nieto made when he launched his election campaign at the end of March.

The 46-year-old president outlined a four-point plan to tackle hunger in 400 of Mexico’s roughly 2,500 municipalities, urging community action, local government responsibility and pledging to strengthen agricultural production in afflicted areas.

However, he did not detail the cost of the plan. “It’s painful and saddening that there are still Mexicans suffering with hunger here in Chiapas, and, it has to be said, in every corner of Mexico,” said Pena Nieto, who took office on December 1.
“This is not a handout program; it’s not just about giving out food to those that need it,” he added. Pena Nieto was speaking in Las Margaritas in the poor southern state of Chiapas, a town that was a hotbed of resistance to the federal government in the 1990s.

Las Margaritas was also a stronghold of the Zapatistas, a leftist revolutionary movement that rose up against the government in 1994, partly in protest against Mexico’s signing of the North American Free Trade Agreement (NAFTA) with the United States and Canada.

Pena Nieto has won praise for pushing hard on a variety of reforms since taking office, but analysts were not convinced on Monday, saying the anti-hunger plan was vague and seemed much like existing policies. “I would say it’s a disappointment because it’s very thin on details,” said Susan Parker, an economist at Mexico City’s CIDE university, adding that it failed to address what would happen to older poverty-fighting programs.

“It’s a rehash,” said Jorge Villasenor, an economist at the University of Guadalajara. “He’s just dusted off policies that have been around since the mid-90s.” Pena Nieto pushed the 2013 budget through Congress barely a week after taking office and gained broad support for a landmark education bill, having convinced political rivals to sign a pact to work with him on a wide-reaching reform agenda.

Former President Felipe Calderon presided over a sharp rise in poverty, and nearly 60 million Mexicans – or around half the population – are now considered poor. That is roughly the same percentage as in the 1980s. Pena Nieto has praised former Brazilian President Luiz Inacio Lula da Silva’s successful social policies, which lifted around 20 million people out of poverty between 2003 and 2009.