ISFAHAN, Iran — Trading the banks of Vienna’s Blue Danube for the muddy waters of the Zayandeh Rood river of Isfahan, OPEC is meeting in Iran for the first time since the Islamic revolution toppled the monarchy and brought in the rule of the clerics.

The gathering in Iran is a testimony to a consensus that has shaped a rare unity among the Gulf oil producers in the cartel over the past five years. This rapprochement helped set in motion the group’s most successful period since its creation in 1960.

Meeting here for the first time in 34 years, oil ministers from Gulf monarchies like Saudi Arabia and Kuwait – once bitter opponents of Iran’s radical regime – will be rubbing shoulders with representatives from Nigeria or Libya, Indonesia and Algeria. In the absence of a government, Iraq is sending a senior adviser from its Oil Ministry.

The Isfahan meeting also comes at a time of acute diplomatic tension with the United States over Iran’s nuclear program. The potential for a confrontation over Tehran’s alleged development of nuclear weapons is raising concerns in oil markets.

Because most producers are producing close to full capacity, many analysts worry that any disruption in supplies, either through war or accident, could send prices surging to $80 a barrel or more.

But the conference on Wednesday is not expected to bring much relief to unsteady oil markets because so far there appears to be a difference of opinion within OPEC on what it will do.

Saudi Arabia, in a statement carried by the Saudi Press Agency, indicated that the cartel should increase its quota by 500,000 barrels above its current ceiling of 27 million barrels a day to help ease prices.

But the oil minister of Iran, Bijan Zanganeh, indicated that more supplies were not needed. He told reporters here Monday that although OPEC felt pressured with prices above $50 a barrel, “everything is not in our hands.”

The 11 members of the Organization of Petroleum Exporting Countries are currently producing 29.5 million barrels a day. That includes production from Iraq, which is not subject to a quota. Some members have been producing above their ceiling to help bring prices down.

Crude oil prices have more than doubled over the past two years and hover around their record highs of $55 a barrel in New York. On the New York Mercantile Exchange on Monday, oil for April delivery rose 52 cents to $54.95.

As OPEC delegates file into Isfahan, their options will be limited. Last week, Sheik Ahmad al-Fahd al-Sabah, the OPEC president who is also Kuwait’s oil minister, said that the cartel was “concerned” about recent price increases “despite the fact that the market is well-supplied and global crude oil stocks have continued to build.”

Some of the group’s largest members seem to be acknowledging that prices will remain high this year. At its last meeting, OPEC dropped a price target of around $25 a barrel that had become largely irrelevant, but it has not chosen a new target.

Instead, Ali al-Naimi, Saudi Arabia’s oil minister, said he expected crude oil prices would remain between $40 and $50 a barrel for the rest of the year.

To be sure, OPEC has also been helped by factors outside of its control. Global oil demand, for example, is growing faster than expected this year, according to estimates by the International Energy Agency released Friday. They showed that demand should be 84.3 million barrels a day in 2005, 1.81 million barrels a day more than last year. Around the world, investments in new exploration, drilling, capacity expansion, pipelines and transportation and refineries have failed to keep pace. That has led to the current tightness in the system.

The insurgency in Iraq, political tension over control of the Russian oil industry and unrest in Nigeria and Venezuela have also contributed to high prices.

The potential for a military confrontation over Iran’s nuclear program is also affecting oil markets. While Europeans are pursuing negotiations for Tehran to renounce any nuclear military program, the United States has not ruled out the use of military force in the country.

“War drums are beating very lightly at this point,” said Vincent Lauerman, editor in chief of Geopolitics of Energy, an industry newsletter based in a Calgary. “But American strikes would have terrible ramifications on the oil market. The Iranians would be willing to make the Americans pay for what might be done to them and shut off their production. They’ve shown that in the past.”

PFC Energy, a Washington-based energy consultant, said in a report last week, “It is clearer by the day that the oil complex is facing intense bullish pressures from a rash of factors: strong demand, low excess supply capacity and economic and political forces that are redoubling interest in commodity investments. These forces are going to create further turbulence in the oil system.”

PFC forecast that prices would average $51.50 a barrel this year and might rise above $60 a barrel.

After the Asian economic slowdown of the late 1990s led to a collapse in prices that pushed the cost of a barrel of oil to $10, the main rivals inside OPEC – Iran and Saudi Arabia – decided that something had to be done. After a series of secret meetings, they agreed to work together to prop up prices. The result has been a tripling of crude-oil prices.

Thanks mostly to a new generation of pragmatic oil ministers, like Zanganeh of Iran and Naimi of Saudi Arabia, OPEC has been turned into a mostly business-like organization seeking the highest returns.

“The situation has changed in the region,” said Hossein Kazempour Ardebili, the Iranian representative on OPEC’s board of governors. “Ideology has no place in our negotiations within OPEC. All players are competitors on the market, but before that they are partners within OPEC.”

This is not to say that OPEC nations have resolved all their differences. For example, Saudi Arabia and Iran have been split for months over whom to pick as OPEC’s next secretary general. Members still argue and bicker over quotas or prices.

But over the past five years, they have patiently rebuilt OPEC’s cohesiveness and discipline.

President Mohammad Khatami, the moderate Iranian leader who sought to break his country’s international isolation since coming to power in 1997, helped achieve the policy shift. It was during his visit to Saudi Arabia two years after his election, the first trip by an Iranian cleric across the Gulf, that Khatami officially sealed Tehran’s new alliance with Crown Prince Abdullah of Saudi Arabia.

In its early days, Iran’s Islamic revolution threatened to spread to the region and topple the region’s conservative Arab monarchies. The Iran-Iraq war had split OPEC and led to a decade-long period of low prices as few countries respected their production quotas.

The organization’s revival was cemented during a meeting of OPEC heads of state in Caracas in March 2000, at the instigation of Venezuela’s president, Hugo Chávez.

“Since that summit, OPEC has put aside political questions and decided to focus on prices and nothing else,” said Pierre Terzian, an energy specialist who runs the Paris-based consulting firm Petrostrategies. “That depoliticization of debates has made OPEC remarkably efficient.”