"It is very clear, for example in the case of China, that their energy interest in Iran gives them a stake in the game," said Ian Bremmer, the president of Eurasia Group, a political risk consultancy based in New York. "Their position is much more engaged here because of their energy policy."

Oil-rich countries, of course, have long used their resources to expand alliances, make new friends or punish adversaries. Nor is energy diplomacy something new for Iran; in the 1970's when the shah ruled Iran, the country was very active in using its oil to build up political support, particularly with the West.

But Iran's Islamic regime is finding that its oil weapon can be a double-edged sword.

With the bulk of the world's oil reserves concentrated in the Persian Gulf and production elsewhere slowly waning, Iran knows that it has time on its side.

"The world will be consuming growing amounts of oil and only five or six countries can supply this," said Mehdi Hashemi, a son of Iran's former president, Ali Akbar Hashemi Rafsanjani, who maintains ties with the oil ministry. "Iran is one of them."

In 1995, Mr. Rafsanjani, who was president then, even tried to lure the United States into improving relations by granting Houston-based Conoco a $1 billion oil development deal.

But the strategy backfired. Ten days after the announcement, President Clinton banned American companies from contributing to Iran's oil sector.

The following year, Congress passed the Iran-Libya Sanctions Act, or ILSA, which threatened penalties against American and non-American companies investing more than $20 million in Iran and Libya's energy sectors.

"Whenever Iran has wanted to get closer to a country it has used its oil diplomacy," said Siamak Namazi, a managing director at Atieh Bahar, the Tehran consultancy. "But the history of American-Iranian relations has been that when one opens the window, the other nails it shut."

Iran is counting on outside help to bolster its stagnating production. After nearly two decades of isolation, the clerical rulers of Iran have realized they cannot afford the massive expansion and modernization the industry needs without capital and expertise from abroad.

Since the mid-1990's, foreign investors, mainly European and Asian companies, have poured about $15 billion into Iran's oil and gas industry. But the country's energy resources remain tired and have never really recovered from the Islamic revolution of 1979.

After the clerics toppled the shah, they cut their oil output by two-thirds to demonstrate their resolve to sever ties with the West; the Iranian industry then became a prime economic target in the eight-year war against Iraq. But when Iran signaled it was once again ready to open up access to foreigners, the United States imposed sanctions against its oil sector.

From a peak production of six million barrels of oil a day in 1974, Iran's oil output slumped to two million barrels in the early 1980's, and has since stabilized at around four million barrels a day, or 5 percent of the world's output.

"Iran has obviously suffered from the departure of the expatriates, from sanctions, from poor management, and from political interference," said Manouchehr Takin, an analyst at the Center for Global Energy Studies in London.

Iran's current expansion plans call for increasing oil production to 5.5 million barrels a day by 2010. But since fields have an annual rate of decline of 200,000 barrels a day, Iran actually needs to find an additional 2.5 million barrels a day - as much as what neighboring Kuwait produces - if it wants to meet its target.

Given the country's restrictive investment rules and the tense political environment, foreign investors are not rushing in. Recently, Lord John Browne, the chief executive of BP, angered Iranian oil officials when he said that to do business with Iran at the moment would be "offensive to the United States and therefore against BP's interests."

Energy has been at the center of Iranian politics ever since the first foreign concession was granted in 1901 to William Knox D'Arcy, an English businessman. It still holds a central part in the nation's recent history - from the creation of the Anglo-Persian Oil Company, the nationalization of the oil sector in 1951, and the CIA-led coup to topple the leftist prime minister, Mohammad Mossadegh, two years later.

The question remains very much alive today in Iran, where foreign ownership of petroleum assets is constitutionally prohibited and where energy policy is vigorously debated in Parliament.

"It's true throughout the Middle East, but the public here is very emotional about the oil issue," Mr. Takin said. "In Iran, the memories of the past are still very vivid."

But as with any high-stakes business, Iran's energy diplomacy is partly a bluff. For many oil executives here, the decisive coup would be, as it tried in 1995, to attract American companies back - but this time lock them in.

"We've been in this business for over a hundred years," said Hossein Kazempour Ardebili, a senior adviser to Iran's foreign and oil ministries. "Security of supplies is our bread and butter. If the United States is looking for security of supplies, Iran is an inevitable partner."