A key lawmaker participating in the congressional probe of the U.N. Oil-for-Food (search) scandal says he's fed up with what he described as the lack of cooperation coming from the United Nations (search).

Rep. Joe Barton, the chairman of the House Energy and Commerce Committee, wrote to Mark Malloch Brown (search), chief of staff to U.N. Secretary General Kofi Annan (search), on Tuesday, disputing recent comments Brown made indicating that the international organization is cooperating with congressional investigators on getting to the bottom of the Oil-for-Food controversy.

"In light of the U.N.'s repeated refusals to provide this committee with specific relevant documents and witnesses, the assertion that 'we are cooperating' with congressional investigations is puzzling, if not disingenuous," Barton, a Texas Republican, wrote in the letter. Read the letter by clicking here.

The House Energy and Commerce Committee is one of five congressional committees probing the $60 billion Oil-for-Food program.

Barton noted that several committees made requests seeking cooperation from the United Nations but they received responses indicating that various documents and witnesses relevant to the investigation are being limited to the Independent Inquiry Committee headed by Paul Volcker (search). In one request from Oct. 18, 2004, lawmakers asked Annan for internal program audits and other "records relating to alleged bribes, kickbacks and surcharges under the program."

Annan responded by saying he has handed over all such documents to Volcker's panel and that he was "not in a position to accede to your requests for documentation" and that the House committee should instead ask Volcker directly for such documents. Barton said that he again requested information relating to "alleged abuses" under the program but that Edward Mortimer, a U.N. spokesman, responded on Dec. 9 by telling the committee to get the information from Volcker.

Brown said in news interviews last Friday that the documents requests by Congress were released to them "some weeks ago," but noted that there is often dispute during such investigations between a special prosecutor or investigator and the congressional committees also probing the issue.

"That's what's happening here, but we are cooperating," said Brown, who is visiting Capitol Hill on Wednesday to discuss Oil-for-Food with some lawmakers.

But Barton's taking issue with that, saying there is no special prosecutor in the Oil-for-Food probe and that "there is no credible threat that the Volcker panel might compromise a congressional inquiry, or vice versa."

"In fact, as noted above, this committee has requested far more from the U.N. than merely the internal audits, which the Volcker committee released to the public several weeks ago," Barton continued, but those documents haven't been made available.

Barton asked Brown to "reconsider its position of asserting cooperation in public while withholding it in practice."

"No valid reason exists for the United Nations to refuse the United States Congress prompt access to essential information on this important matter," Barton wrote.

A report released by Volcker's committee on Feb. 3 pointed to significant management problems within the United Nations surrounding the Oil-for-Food program. The report said that Benon Sevan (search), the former program chief, had "seriously undermined" the integrity of the United Nations. At least two more reports are expected from Volcker's group before the end of the year.

Manhattan District Attorney Robert Morgenthau has launched a criminal probe into Sevan, who has been accused of receiving about $1 million worth of lucrative oil vouchers through the program.

FOX News' Eric Shawn and Jonathan Wachtel contributed to this report.