WASHINGTON, June 4 (Reuters) -

A group of Democratic U.S. House lawmakers asked the Justice Department on Tuesday to probe allegations of antitrust behavior among U.S. oil producers and OPEC and said the two largest U.S oil companies have been conspiring to keep fuel prices high.

Nine House of Representatives Democrats, in a letter to Attorney General Merrick Garland, cited a complaint by the Federal Trade Commission (FTC) in May that alleged that the former CEO of Pioneer Resources engaged in an "outrageous scheme... to coordinate pricing between U.S. oil companies and foreign producers."

The letter, led by Jerrold Nadler, the top Democrat on the House Judiciary Committee, also cited the big profits earned last year by Exxon Mobil Corp and Chevron, the two largest U.S. oil companies.

"Major oil producers appear to be colluding with each other and foreign cartels to keep prices high, padding their profits at the expense of American consumers," the lawmakers said.

"We urge you to use the full authority of the Department of Justice to investigate and, where necessary, prosecute this anticompetitive conduct," the letter said.

On Exxon and Chevron, the lawmakers said: "But apparently, instead of passing those profits through to consumers in the form of cheaper products, the oil giants have been lining their own pockets while conspiring to keep prices high."

Exxon and Chevron did not immediately respond to a request for comment.

The Organization of the Petroleum Exporting Countries also did not immediately respond to a request for comment.

In addition to Nadler, the letter was signed by Henry Jackson, Pramila Jayapal and seven other Democrats.

Last month, the FTC approved Exxon Mobil's $60 billion purchase of Pioneer Natural Resources, but barred former Pioneer CEO Scott Sheffield from Exxon's board on allegations he attempted to collude with OPEC to raise oil prices.

Late last month, U.S. Senate Majority Leader Chuck Schumer and 22 of his Democratic colleagues

sent a similar letter

to Garland.

Sheffield coordinated efforts with U.S. shale oil producers to constrain their output and raise energy prices, the FTC said on May 2.

Sheffield, known for his long tenure and frank comments on industry output and spending, used his influence "to align oil production across the Permian Basin in West Texas and New Mexico with OPEC+," a production group that includes Russia as well as OPEC members, the FTC said at the time.

Sheffield's lawyers have referred to comments made last month, when Sheffield asked the FTC to dismiss the ban. "At no time did government officials and Mr. Sheffield exchange competitively sensitive information," said Sheffield's counsel Cleary Gottlieb Steen & Hamilton. (Reporting by Timothy Gardner and Susan Heavey; Editing by Leslie Adler)