Mexican lawmakers called on authorities on Monday to investigate allegations of bribery at the Mexico unit of Wal-Mart Stores Inc, even though prosecutors said it may not be a federal matter.

Mexico's attorney general would only look into the accusations against Wal-Mart de Mexico (Walmex) if asked to do so by the Ministry of Finance or Ministry of the Economy, a government official said on condition of anonymity.

Officials at those ministries could not immediately be reached for comment.

Two opposition left-wing senators on the Senate's Banking and Public Credit Commission have called for an investigation.

If licenses were given out where they shouldn't have been, there's fraud not only in the cities where that happened, but also there could have been fiscal fraud, Francisco Javier Castellon of the Party of the Democratic Revolution (PRD) was quoted as saying in a report on Monday in the newspaper Rumbo de Mexico.

Leftist presidential candidate Andres Manuel Lopez Obrador said on Sunday that the allegations showed the government was rotten, and he expressed dismay that the case was so far only being investigated outside Mexico.

Wal-Mart said on Saturday that it had begun an investigation into its compliance with the U.S. Foreign Corrupt Practices Act (FCPA) last fall. It said that it had disclosed the probe to the U.S. Department of Justice and U.S. the Securities and Exchange Commission.

While political pressure for action may increase in Mexico ahead of presidential elections on July 1, corruption-weary Mexicans barely raised eyebrows when they read of the bribery allegations in local newspapers.

The New York Times reported over the weekend that Wal-Mart had silenced an internal investigation of hundreds of suspect payments worth more than $24 million made to grow its business in Mexico.

Bribery and corruption is pervasive in Mexico, where the justice system is weak and lower-level public sector workers earn relatively low salaries. One study last year by Transparency International showed that Mexican companies were perceived to be the third most likely behind those in China and Russia to pay bribes abroad.

Walmex shares were down almost 12 percent at midday on Monday after the New York Times report. The company said in a statement that it has taken steps to boost internal controls to make sure it is FCPA-compliant, according to a statement.

Walmex is due to report first-quarter financial results after the market closes on Monday.

(Additional reporting by Mica Rosenberg)