The senator leading an investigation into the United Nations' Oil-for-Food program in Iraq is calling on Secretary-General Kofi Annan (search) to resign.

Writing in Wednesday's Wall Street Journal, Sen. Norm Coleman (search), R-Minn., argues that Annan should step down because "the most extensive fraud in the history of the U.N. occurred on his watch. In addition, and perhaps more importantly, as long as Mr. Annan remains in charge, the world will never be able to learn the full extent of the bribes, kickbacks and under-the-table payments that took place under the U.N.'s collective nose."

Annan's office declined to comment Tuesday night.

The humanitarian Oil-for-Food program (search), which began in 1996, allowed Iraq to trade oil for food, medicine and other necessities that became scarce under strict U.N. economic sanctions imposed after the Gulf War. The program was credited with preventing widespread starvation.

Two weeks ago, Coleman's Permanent Subcommittee on Investigations said it had uncovered evidence that Saddam Hussein's government raised more than $21.3 billion in illegal revenue by subverting U.N. sanctions against Iraq, including the Oil-for-Food program.

"Mr. Annan was at the helm of the U.N. for all but a few days of the Oil-for-Food program, and he must, therefore, be held accountable for the U.N.'s utter failure to detect or stop Saddam's abuses," wrote Coleman.

The subcommittee's ranking Democrat, Carl Levin (search) of Michigan, has worked closely with Coleman on the investigation, but said Tuesday night he didn't see a need for Annan to resign.

"While I believe that one or more specific individuals working with the Oil-for-Food program at the U.N. may have acted improperly, I have seen no evidence of impropriety whatsoever on the part of Kofi Annan," Levin said in a statement issued to The Associated Press.

In a telephone interview Tuesday night, Coleman said: "Any private company would have asked for his (Annan's) resignation. But the members of the board, in this case Security Council members China, Russia and France, have all benefited from Saddam being in power."

Coleman said another factor in asking for Annan's resignation was the secretary-general's refusal to give his subcommittee access to the U.N.'s 55 internal audits and interviews with U.N. staff members. Annan has argued that doing so would interfere with an internal investigation by former Federal Reserve Chairman Paul Volcker.

Coleman stressed that in calling for Annan's resignation, he wasn't coming to any conclusions about his involvement in the Oil-for-Food program.

"I'm not Kofi's boss," Coleman said.

He called Annan's resignation "inevitable," especially in light of this week's revelation that Annan's son, Kojo Annan, received $30,000 a year for over five years from a Swiss-based company under investigation in connection with suspected corruption in the Oil-for-Food program.

Coleman stopped short of saying he would move to cut U.S. funding for the U.N. if Annan stays on.

"I'm not prepared to say we need to step back from our (financial) commitments to the U.N," he said. "But ultimately, if our efforts are thwarted, I think that issue does come into play. The consequence of the path we're on right now is not a good path for the United Nations."

Coleman said he didn't ask any other senator to join his call for Annan's resignation, but said he had "no doubt" that many senators will support the effort.