(Recasts with five people killed, details, paragraphs 1-4)

MEXICO CITY, Aug 11 (Reuters) - Five people were killed after a Pemex gas pipe burst into flames on the outskirts of the industrial city of Monterrey in northern Mexico on Tuesday, the state oil giant said on Twitter.

A blow from a piece of machinery belonging to a contractor caused the blaze, the company tweeted. The fire was still burning but was "under control."

The five people killed were employees of the contractor, Pemex said without identifying the firm.

Pemex said it cut off the flow of gas to the pipe section where the fire occurred in Garcia, a municipality in the state of Nuevo Leon, after it detected a rapid loss of pressure.

Nemak, the auto parts unit of Mexican conglomerate Alfa, said in a statement personnel from its nearby plant had been evacuated even though the blaze occurred outside its installations.

(Reporting by Tomas Sarmiento and Alexandra Alper; Editing by Leslie Adler and Paul Tait)