Two U.S. senators investigating the U.N. Oil-for-Food (search) program have told U.N. Secretary-General Kofi Annan (search) in a letter that they were "troubled" by his decision to withhold documents or witness testimony from lawmakers.

Sens. Norm Coleman (search) of Minnesota and Carl Levin (search) of Michigan sent a letter to Annan Tuesday in which they blasted Annan for "affirmatively preventing" their congressional panel from getting requested information.

"They are not providing access to U.N. personnel, not providing access to U.N. internal audits," Coleman told FOX News.

Coleman and Levin — respectively, the chairman and ranking minority member of the Senate Governmental Affairs investigations subcommittee — want to know how Saddam Hussein was able to pocket an estimated $11 billion through payoffs and oil smuggling.

Click here to read the senators' letter as well as other related correspondence (pdf).

The lawmakers are asking Annan to hand over documents outlining why the consulting firm Lloyd's Register, a London-based company, was dropped in favor of a firm his son, Kojo Annan (search), once worked for. A Lloyd's official asked for U.N. permission to cooperate with the panel but was denied.

"The organization has no objections to a contractor responding to authorized subpoenas for records, documents, tapes or other materials, subject to certain conditions. I note, however, that in the present case, the requested cooperation would be entirely on a voluntary basis … Under the circumstances, the U.N. does not authorize the provision of materials or information relating to its contract with Lloyd’s Register," Bruce C. Rashkow (search), director of the U.N. general legal division, wrote to a Lloyd’s official on Aug. 31.

Both Annan and his son's company, Swiss-based Cotecna Inspection SA, have denied any wrongdoing, but Annan has also not yet made available audits that could detail who exactly was getting money. The Oil-for-Food program was started in late 1996 as a way to let Iraq sell some oil to allow humanitarian goods reach Iraqis but investigators now believe the program was a multi-billion dollar corruption scheme.

Asked about the senators’ letter, Annan’s communications director said he would look into the claims that his boss is not cooperating. The spokesman said the situation was "very awkward" and he called the senators’ communication "a troubling letter."

The Senate panel also wants to talk to Benon Sevan (search), the former head of the Oil-for-Food program. Sevan has been accused of receiving bribes from Saddam but he denies the charges.

"We have a number of questions we want to ask the individual in charge of the program and the most serious question that has been reported: ‘Why did you get these vouchers? What was the reason you got them in the first place?'"

Sevan has promised to cooperate with investigators and has denied any wrongdoing. But he has yet to agree to testify before congressional panels.

Coleman told FOX News he's angry that the United Nations has so far tried to block his investigation. Senate hearings start Monday.

The CIA and others say Saddam rigged a global network of bribes, payoffs and kickbacks, including hundreds of fake front companies, to earn his billions. The United Nations is conducting its own investigation, but that report is not due until next year.

FOX News' Eric Shawn contributed to this report.