U.S. and Mexican negotiators have failed to reach agreement on the purchase of Mexican natural gas by the United States, the Foreign Ministry announced tonight.

Foreign Minister Jorge Castaneda and Undersecretary of State Warren Christopher were unable to negotiate an agreement in two days of discussions. Christopher, number two man in the State Department, had delayed his return to the United States by a day in effort to settle the issue.

The Foreign Ministry statement said that there are "still differences about prices and other aspects" outstanding between the two countries. It did not elaborate on the differences, and U.S. sources were not available for comment.

The failure of the gas negotiations may cause President Jose Lopez Portillo to put off his scheduled visit with President Carter in Washington next month, according to well-informed Mexican government sources.

The negotiations have become increasingly sensitive with each side charging the other with bad faith. U.S. officials have said that Mexico reneged on a verbal agreement that President Lopez Portillo reportedly made setting the price at $3.40 per thousand cubic feet.

The discussions have been further exacerbated by U.S. attempts to get Mexico to crack down on the illegal immigrants from that country who are entering the United States in large numbers.

Mexico also reacted angrily when a U.S. official said that Washington would ask Mexico to help pay for the damages to U.S. coastal areas caused by oil from a runaway Mexican well.

The two countries have reportedly settled part of the natural gas issue. Agreement is said to have been made on the sale of 300 million cubic feet daily with the price to be readjusted every three months. The initial price, however, has yet to be settled.