More than 40 U.S. lawmakers have written a letter to President Donald Trump expressing concern that the Mexican government is threatening U.S. energy companies' investments and market access, as well as undermining a regional trade deal.

The letter, dated Oct. 22 and seen by Reuters on Oct. 23, argued that reports showed Mexican President Andres Manuel Lopez Obrador is providing preferential regulatory treatment for state oil firm Petroleos Mexicanos, known as Pemex, and delaying or canceling permits for U.S. energy companies.

Four congressional sources confirmed the letter's authenticity.

Lopez Obrador, who took office in December 2018, argues that past governments fronted by corrupt politicians had skewed the Mexican energy market in favor of private interests at the expense of the public.

The letter noted that in a recently leaked memo, Lopez Obrador urged Mexican regulators to use all available means to protect Pemex and the Comision Federal de Electricidad (CFE), Mexico's state electricity company.

"These efforts violate and contradict the spirit, if not the letter, of the USMCA (United States-Mexico-Canada Agreement), an agreement among whose primary objectives are to promote growth among the participant countries," said the letter.

Negotiated at the behest of Trump, the USMCA from July replaced the 1994 North American Free Trade Agreement (NAFTA).

Signed by 43 members of Congress from both parties, many from states with powerful energy sectors, the letter said the actions of Lopez Obrador and his government undermined billions of dollars of U.S. investment in Mexico.

The U.S. lawmakers said the actions demonstrate "a pattern of obstruction" and urged Trump to find a resolution with Mexico's government to preserve the market conditions set out under a 2013-14 energy reform that opened up the sector.