A huge blast rocked the headquarters of Mexican state-owned oil giant Pemex on Thursday, killing at least 25 people, injuring 100 and leaving rescuers scrambling to find survivors into the night.

Officials said the blast ripped through an annex at around 3:40pm, causing severe damage to three floors. Witnesses said a roof connecting the annex to the tower collapsed. Thousands of people were evacuated.

Interior Minister Miguel Angel Osorio Chong told a news conference that 17 women and eight men had died, and that 101 people had been injured.

A spokesman for the civil protection agency said there was an apparent "accumulation of gas" in an electrical supply room, but the exact cause of the blast has yet to be confirmed.