MEXICO CITY (Reuters) - At least seven people were injured by an explosion after an attempt to illegally tap a pipeline belonging to the Mexican state oil monopoly Pemex near Mexico City, officials said on Sunday.

The early morning blast at the crude oil pipeline occurred in Tonanitla about 40 km (25 miles) north of the capital, and was now under control, Pemex said on its Twitter account.

The government of the state of Mexico, a populous region that borders much of the capital, said seven people were injured by the explosion. Five were police and two worked for the fire brigade. It was not known who caused the blast.

Pipeline taps are a frequent menace to Pemex, which loses hundreds of millions of dollars a year to oil theft.

A symbol of Mexican self-sufficiency, Pemex is struggling to reverse a drop in production, which has fallen to about 2.5 million barrels per day from a peak of 3.4 million in 2004.

The government is due to unveil a plan to open up the company to more private capital by early September.

(Reporting by Miguel Gutierrez; Editing by Bill Trott)