The U.N. Security Council unanimously approved a resolution Wednesday to back an investigation into the troubled Iraqi oil-for-food program.

The probe will look into alleged corruption within the United Nations and how Saddam Hussein (search) and his allies allegedly siphoned off billions of dollars that were meant for the Iraqi people.

"I can't guarantee the investigation will not be a cause of some controversy," Paul Volcker, appointed head of the inquiry, said Wednesday. "All I can say is we'll make the investigation as complete as we can make it."

Volcker, the respected former chairman of the U.S. Federal Reserve, said his investigative team will issue a report in three months but he anticipates the inquiry lasting far longer.

"The three month update is an update — we will describe where we are, it is not a deadline for a substantive report," he said. "I would not expect a conclusive report in three months."

Volcker said his first priority, after hiring a staff, is to look into accusations of internal U.N. corruption and then look at the overall management of the program that has led to allegations of bribes, kickbacks and illegal contracts that cost an estimated $10 billion.

Volcker had requested the resolution before accepting the job to assure that the U.N. investigation wouldn't exempt U.N. Secretary-General Kofi Annan's (search) office.

Annan's son once worked for a program consultant but denies any wrongdoing. Sources say the resolution says that any illicit activity by U.N. officials, personnel and contractors is "unacceptable."

Last week the Russians said there was no need for a resolution but have since retracted their objection. Critics said the Russians were against it because they made billions off Saddam and do not want the truth to come out.

"There are many countries, individuals ... who do not want the truth to come out ... they have cheated the system," Claude Hankes-Drielsma (search), advisor to Iraq's Governing Council told Fox News, citing Russia and France as two countries that were "significant beneficiaries" of Saddam.

A list of people who allegedly got vouchers to sell Iraqi oil included a former Russian ambassador to Baghdad and the former head of the program itself, Benon Sevan.

Sevan has denied the allegations and he's now on leave pending his retirement.

But experts who have examined the program say only an independent investigation will uncover the truth.

"Mr. Benon Sevan's name appears on several records, which I have seen personally in ministries in Baghdad ... and one has to ask, why is that?" said Hankes-Drielsma. "The same applies to the former French ambassador to the U.N. who appears on the list, the son of a Russian ambassador on the list. And I can go on for quite a while."

Volcker said he hasn't seen the records yet but they will be probed as part of the investigation.

"I'm told substantial records exist in Iraq particularly on the oil-for-food program," he said Wednesday. "I haven't examined those records... Obviously there are people there who may be trying to hide things, I'd be surprised If they're not."

The corruption claims — a major embarrassment for the United Nations — surfaced last January in the Iraqi newspaper Al-Mada, which published a list of about 270 former government officials, activists and journalists from more than 46 countries suspected of profiting from Iraqi oil sales under the U.N. program.

The U.S. General Accounting Office (search), Congress' investigative arm, estimated last month that the Iraqi government pocketed $5.7 billion smuggling oil to its neighbors and $4.4 billion extracting illicit surcharges and kickbacks on otherwise legitimate contracts.

Annan launched an internal inquiry into the oil-for-food program in February but canceled it to allow an independent examination covering governments and companies that signed contracts with the United Nations or Iraq.

Fox News' Eric Shawn, Amy Sims and The Associated Press contributed to this report.