A powerful explosion has rocked the Mexico City headquarters of state-owned oil giant Pemex, killing at least 25 people and injuring more than 100.
The blast on Thursday afternoon shattered the lower floors of the 54-storey tower, throwing debris into the streets and sending frightened workers running outside.
Emergency services assisted by dogs are searching for an unknown number of people trapped in the rubble at the base of the tower and the cause of explosion is being investigated.
The Pemex Executive Tower houses hundreds of employees. In September last year, 30 people died in an explosion at a Pemex gas plant in northern Mexico, the BBC reports.
Thursday's explosion in the lower floors of the building happened as shifts were changing in the afternoon, making the area particularly crowded.
Police cordoned off the streets around the high-rise, located in a busy commercial area of Mexico City. Pemex chief executive Emilio Lozoya Austin has cut short a business trip to Asia and is returning to Mexico.
Relatives of employees gathered outside the building in search of information. Some are said to have tried to reach employees on their mobile phones, but had no reply.
"The place shook, we lost power and suddenly there was debris everywhere. Colleagues were helping us out of the building," eyewitness Cristian Obele said.
A government official told Reuters the blast may have come from a gas boiler that exploded in an adjacent Pemex building.
Earlier on Thursday, Pemex had reported problems with the electricity in the building in a message on Twitter. It later confirmed that an explosion had taken place "in the B2 building of the administrative centre".
Pemex says its operations will continue to run normally - and commercial and financial obligations will continue to be met - despite the blast.