Rescue workers pulled out more bodies from debris at the headquarters of Mexican state oil giant Pemex, after a powerful explosion killed at least 33 people.

The explosion also threw a spotlight onto the state-run company's poor safety record.

Scenes of confusion and chaos outside the tower block in Mexico City have dealt another blow to Pemex's image, just as Mexico's new president is seeking to court outside investment for the 75-year-old monopoly.

The blast occurred at a Pemex building next to the 50-storey skyscraper and senior officials said 33 people had so far been confirmed dead.

A further 121 were injured, said the company's chief executive, Emilio Lozoya.

Officials have been unable to say how many people may still be trapped in the wreckage of the office block, which remains cordoned off.

A military paramedic at the scene said there are likely many and he expected the death toll to keep rising.

Mr Lozoya said it was not clear what caused the mid-afternoon explosion, which has been the subject of speculation ranging from a bomb attack, to a gas leak, to a boiler blowing up.

"A fatal incident like yesterday's cannot be explained in two hours, we are working with the best teams in Mexico and from overseas, we will not speculate," he told a news conference.