Oil prices will keep rising over the next two decades unless the oil-rich nations of the Middle East and North Africa substantially increase investments in their energy sectors, according to a report released on Monday by the organization that represents energy- consuming nations.

The World Energy Outlook, an annual publication that outlines long-term forecasts by the International Energy Agency in Paris, calls on producers like Saudi Arabia to expand investment to meet a projected 40 percent jump in oil demand by 2030.

To meet these needs, the agency estimates that $3 trillion is needed worldwide for oil exploration, development and refining during the next 25 years. Middle East and North African producers will need to double their investments to $23 billion a year, or more than $614 billion twice the average annual amount spent over the past decade in these regions, according to the report.

But the energy agency, which represents the interests of oil- consuming nations, acknowledged that some countries might not be willing to invest in higher output. Instead, it said, they may seek to curb growth in their output, leading to higher prices.

"If these countries do not increase their investments substantially, we will end up with difficulties on the energy markets," said Fatih Birol, the energy agency's chief economist. "We may end up with much higher prices," he said before the release of the report. "The issue in the oil business today is not the reserves but the money, the investment decisions, the investment climate," Birol said.

The energy agency acts as a policy adviser and forecaster for industrialized nations, and its reports are widely read by governments worldwide and by the energy industry. But what some see as the agency's political bias toward oil-consuming nations sometimes leads it to produce assessments that some view as unrealistic.

For example, under the agency's model, Saudi Arabia would need to bring its production up to 18 million barrels a day by 2030 to meet the jump in demand. Today, it produces about 10.5 million barrels a day.

Saudi Arabia, which does not allow foreign investment in its oil exploration and production sectors, has disputed such high forecasts in the past. Its own public investment plans call for increasing output to 12.5 million barrels a day by 2009. Saudi officials have said they could increase production to 15 million barrels a day but no more and could sustain that level for 50 years.

The issue of access to reserves, which executives highlighted last year, has been overshadowed recently by the debate over whether global oil production is at or near its peak. Birol said the issue was whether investments were made, not whether reserves existed.

"Saudi Arabia has the oil reserves and the domestic capital to boost their production," Birol said. "But they might not want to increase their investments to boost production. They may want to influence the world price environment."

Oil-producing nations are often wary of allowing major foreign investment in their national oil sectors. They may fear creating a glut that would reduce lower prices, or they may doubt bullish projections for demand. For the first time, Birol said, the agency has produced an alternative forecast that factors in a possible drop in investment by the Organization of the Petroleum Exporting Countries as a whole and by Saudi Arabia, the world's top producer. The figures are so low, in fact, that they are below the kingdom's longer-term targets. In that so-called deferred scenario, Saudi Arabia's output in 2030 is estimated at 14 million barrels a day.

It is not the first time that the energy agency has highlighted the need for more investment by OPEC members. The theme has become fairly common for the agency, which has repeatedly asked oil producers to open their energy sectors to foreign companies.