United Nations officials have rejected charges that they are obstructing outside investigations into the Iraq Oil for Food program.  The world body is vigorously defending a decision to withhold evidence from U.S. Congressional investigators.

The head of the U.N. panel investigating alleged fraud in the Iraq Oil for Food program has informed a U.S. Senate subcommittee that he will not turn over evidence or share witnesses until his probe is completed.

Paul Volcker, the former U.S. Central Bank chief appointed by Secretary General Kofi Annan to head the U.N. probe, also rejected suggestions that employees involved in administering the humanitarian program should testify before U.S. Congressional committees.

In a letter to the ranking members of the Senate Investigations Subcommittee, Republican Norm Coleman of Minnesota and Democrat Carl Levin of Michigan, Mr. Volcker warned in particular that the release of potentially misleading and incomplete information could impair his investigation and distort public perceptions.

U.N. spokesman Fred Eckhard said the Secretary General also called both senators in an attempt to straighten out any misunderstandings.

"The Secretary-General called each of the senators, Senator Levin and Senator Coleman last weekend to assure them he wanted to get to the bottom of this and did not want to obstruct investigation, but that he also has, as head of this organization obligations to 190 other countries," he noted.

The Secretary-General's phone calls came after the two senators accused U.N. officials of trying to cover up the oil for food scandal.

Spokesman Eckhard Wednesday flatly rejected the coverup charges, calling them "perverse". He pointed out that the independent U.N. Oil for Food probe had been authorized by the Security Council with the understanding that Mr. Volcker's staff would control all documentation.

"It is the only way Mr. Volcker can conduct an orderly and a thorough investigation, and to view this as somehow covering up when all the relevant information has been turned over to a very trusted individual who has enormous integrity and who is expected to get to the bottom of questions raised about Oil for Food," he added.  "So I think I would say it's perverse to describe this as a coverup."

The U.N.-administered Iraq Oil for Food program operated from 1996 to 2003. It was designed to allow Saddam Hussein's government to ease the burden of international sanctions on the population by selling oil to buy humanitarian supplies.

But evidence uncovered by investigators suggests the Iraqi dictator abused the program, using oil vouchers as bribes to hundreds of people and favored businesses in an attempt to buy influence and have the sanctions overturned.

The Senate investigations subcommittee Monday heard testimony that Saddam's regime may have reaped more than $21 billion from oil kickbacks and smuggling while the sanctions were in place.