Between 100,000 and 300,000 barrels a day of Iraq’s declared oil production over the past four years is unaccounted for and could have been siphoned off through corruption or smuggling, according to a draft American government report.

Using an average of $50 a barrel, the report said the discrepancy was valued at $5 million to $15 million daily.

The report does not give a final conclusion on what happened to the missing fraction of the roughly two million barrels pumped by Iraq each day, but the findings are sure to reinforce longstanding suspicions that smugglers, insurgents and corrupt officials control significant parts of the country’s oil industry.

The report also covered alternative explanations for the billions of dollars worth of discrepancies, including the possibility that Iraq has been consistently overstating its oil production.

Iraq and the State Department, which reports the numbers, have been under relentless pressure to show tangible progress in Iraq by raising production levels, which have languished well below the United States goal of three million barrels a day. Virtually the entire economy of Iraq is dependent on oil revenues.

The draft report, expected to be released within the next week, was prepared by the United States Government Accountability Office with the help of government energy analysts, and was provided to The New York Times by a separate government office that received a review copy. The accountability office declined to provide a copy or to discuss the draft.

Paul Anderson, a spokesman for the office, said only that “we don’t discuss draft reports.”

But a State Department official who works on energy issues said that there were several possible explanations for the discrepancy, including the loss of oil through sabotage of pipelines and inaccurate reporting of production in southern Iraq, where engineers may not properly account for water that is pumped along with oil in the fields there.

“It could also be theft,” the official said, with suspicion falling primarily on Shiite militias in the south. “Crude oil is not as lucrative in the region as refined products, but we’re not ruling that out either.”

Iraqi and American officials have previously said that smuggling of refined products like gasoline and kerosene is probably costing Iraq billions of dollars a year in lost revenues. The smuggling of those products is particularly feared because officials believe that a large fraction of the proceeds go to insurgent groups. Crude oil is much more difficult to smuggle because it must be shipped to refineries and turned into the more valuable refined products before it can be sold on the market.

The Shiite militia groups hold sway around the rich oil fields of southern Iraq, which dominate the country’s oil production, the State Department official said. For that reason, he said, the Shiite militias are more likely to be involved in theft there than the largely Sunni insurgents, who are believed to benefit mostly from smuggling refined products in the north.

In the south, the official said, “There is not an issue of insurgency, per se, but it could be funding Shia factions, and that could very well be true.”

“That would be a concern if they were using smuggling money to blow up American soldiers or kill Sunnis or do anything that could harm the unity of the country,” the official said.

The report by the accountability office is the most comprehensive look yet at faltering American efforts to rebuild Iraq’s oil and electricity sectors. For the analysis of Iraq’s oil production, the accountability office called upon experts at the Energy Information Administration within the United States Department of Energy, which has long experience in analyzing oil production and exports worldwide.

Erik Kreil, an oil expert at the information administration who is familiar with the analysis, said a review of industry figures around the world — exports, refinery figures and other measures — could not account for all the oil that Iraq says it is producing. The administration also took into account how much crude oil was consumed internally, to do things like fuel Iraqi power plants and refine into gasoline and other products.

When all those uses of the oil were taken into consideration, Mr. Kreil said, Iraq’s stated production figures did not add up.

“Either they’re producing less, or they’re producing what they say and the difference is completely unaccounted for in any of the places we think it should go,” Mr. Kreil said. “Either it’s overly optimistic, or it’s unaccounted for.”

Several analysts outside the government agreed that such a large discrepancy indicated that there was either a major smuggling operation in place or that Iraq was incapable to generate accurate production figures.

“That’s a staggering amount of oil to lose every month,” said Philip K. Verleger Jr., an independent economist and oil expert. “But given everything else that’s been written about Iraq, it’s not a surprise.”

Mr. Verleger added that if the oil was being smuggled out of Iraq, there would be a ready market for it, particularly in smaller refineries not controlled by large Western companies in places like China, the Caribbean and even small European countries.

The report also contains the most comprehensive assessment yet of the billions of dollars the United States and Iraq spent on rebuilding the oil and electricity infrastructure, which is falling further and further behind its performance goals.

Adding together both civilian and military financing, the report concludes that the United States has spent $5.1 billion of the $7.4 billion in American taxpayer money set aside to rebuild the Iraqi electricity and oil sectors. The United States has also spent $3.8 billion of Iraqi money on those sectors, the report says.

Despite those enormous expenditures, the performance is far short of official goals, and in some cases seems to be declining further. The average output of Iraq’s national electricity grid in 2006, for example, was 4,300 megawatts, about equal to its value before the 2003 invasion. By February of this year, the figure had fallen still further, to 3,800 megawatts, the report says.

All of those figures are far short of the longstanding American goal for Iraq: 6,000 megawatts. Even more dispiriting for Iraqis, by February the grid provided power for an average of only 5.1 hours a day in Baghdad and 8.6 hours nationwide. Both of those figures are also down from last year.

The story is similar for the oil sector, where — even if the Iraqi numbers are correct — neither exports nor production have met American goals and have also declined since last year, the report says.

American reconstruction officials have continued to promote what they describe as successes in the rebuilding program, while saying that problems with security have prevented the program from achieving all of its goals. But federal oversight officials have frequently reported that the program has also suffered from inadequate oversight, poor contracting practices, graft, ineffective management and disastrous initial planning.

The discrepancies in the Iraqi oil figures are broadly reminiscent of the ones that turned up when some of the same energy department experts examined Iraq’s oil infrastructure in the wake of the oil-for-food scandals of the Saddam Hussein era. In a United Nations-sponsored program that was supposed to trade Iraq’s oil for food, Mr. Hussein and other smugglers were handsomely profiting from the program, investigations determined.

In reports to Congress before the 2003 invasion that ousted Mr. Hussein, the accountability office, using techniques similar to those called into play in its most recent report, determined that in early 2002, for example, 325,000 to 480,000 barrels of crude oil a day were being smuggled out of Iraq, the majority through a pipeline to Syria.

But substantial amounts also left Iraq through Jordan and Turkey, and by ship in the Persian Gulf, routes that could also be available today, said Robert Ebel, a senior adviser at the Center for Strategic and International Studies in Washington.

“Any number of adjacent countries would be glad to have it if they could make some money,” Mr. Ebel said.

Mr. Ebel said the lack of modern metering equipment, or measuring devices, at Iraq’s wellheads made it especially difficult to track smuggling there. The State Department official agreed that there were no meters at the wellheads, but said that Iraq’s Oil Ministry had signed a contract with Shell Oil to study the possibility of putting in the meters.

The official added that an American-financed project to install meters on Iraq’s main oil platform in the Persian Gulf was scheduled to be completed this month.

As sizable as a discrepancy of as much as 300,000 barrels a day would be in most parts of the world, some analysts said it could be expected in a country with such a long, ingrained history of corruption.

“It would be surprising if it was not the case,” said John Pike, director of GlobalSecurity.org, which closely follows security and economic issues in Iraq. He added, “How could the oil sector be the exception?”