NEW YORK — The chairman of the independent committee probing the U.N. oil-for-food program telephoned at least two congressmen to express concern about forcing the testimony of a senior investigator who resigned in protest of findings that he considered too soft on U.N. Secretary-General Kofi Annan, officials said Friday.

The chairman, former Federal Reserve chief Paul Volcker, spoke with the congressmen Thursday about possible plans to subpoena the investigator, Robert Parton. Volcker noted the confidentiality agreement in Parton's contract and the U.N.-appointed committee's diplomatic immunity, said Mike Holtzman, a spokesman for the Volker committee.

Several congressional committees are considering subpoenas for the investigator, said Tom Costa, a spokesman for Rep. Christopher Shays, R-Conn., one of the congressmen Volcker called.

The disagreement highlights the tension between the world body and some members of Congress who believe the secretary-general got off too easily in the report. The report cleared Annan of interfering in the awarding of a $10 million-a-year U.N. contract to the Swiss employer of his son, Kojo Annan, but faulted Kojo for hiding the extent of his own involvement.

The report said Annan didn't properly investigate possible conflicts of interest surrounding the contract, criticizing him for refusing to push top advisers further after they conducted a 24-hour probe relating to his son and found nothing wrong.

The interim report, released March 29, said he didn't violate U.N. rules. The $64 billion oil-for-food program was set up to help Iraqis cope with U.N. sanctions imposed on Saddam Hussein's regime after his 1990 invasion of Kuwait.

The investigators on Volcker's team signed nondisclosure clauses, agreeing not to discuss their work for the committee.

Shays said the United Nations and Volcker's committee had refused to waive immunity for Parton, who resigned in protest this month along with a second investigator, Miranda Duncan.

"This lack of transparency is part of the problem," Shays said in a statement e-mailed to The Associated Press. "We are conferring with lawyers to determine our next steps."

Holtzman said the Volker committee was merely protecting the investigator.

"The committee has done everything possible to be transparent, consistent with conducting a fair and impartial investigation, he said. "We draw the line at exposing an investigator during an ongoing investigation."

Volcker also called Rep. Henry Hyde, R.-Ill., said Hyde's spokesman, Sam Stratman. He said the call was cordial and that two men spoke in general terms and did not specifically discuss the investigator.

The House International Relations Committee, which Hyde chairs, has been among the most assertive of several congressional committees investigating the oil-for-food scandal. Shays chairs the House Government Reform Subcommittee on National Security, Emerging Threats and International Relations.

Mark Pieth, one of Volcker's two colleagues on the committee, told the AP that the two investigators were upset because they believed the report was too soft on the secretary-general.

According to a person close to Parton, who spoke on condition of anonymity at Parton's request, the investigator wrote at least two drafts of the report that were significantly more critical of Annan.

As senior investigative counsel, Parton was responsible for investigations into the procurement of companies under the oil-for-food program and was the lead investigator on allegations of impropriety relating to Kofi and Kojo Annan. Duncan worked on Parton's team.