Iran has become the second-largest supplier of crude oil to the United States, the result of a surge of exports this summer that has pumped more than $700 million into Tehran's war-torn economy, according to a new analysis of U.S. oil imports.

The sudden growth of this economic link between Iran and the United States -- which coincides with an escalating military and diplomatic confrontation between the two nations -- is documented in Commerce Department figures cited by John Roberts, senior adviser of the Middle East Institute in Washington.

"In July the United States imported 19.6 million barrels of Iranian crude oil at a cost of $359 million. Only Nigeria supplied more," Roberts said.

The Iranian shipments amounted to more than 11 percent of U.S. oil imports, while payments from U.S. oil companies provided Iran with about one-third of its total oil revenues, Roberts said. U.S. oil payments to Iran for just the three months ending in July exceeded the annual average in each of the previous three years, he said.

Iran is also trying to arrange direct purchases of at least $40 million worth of oil-field equipment from U.S. companies in an attempt to replace machinery and rebuild facilities damaged by recent Iraqi air attacks, Roberts said.

"Neither side yet seems to appreciate the magnitude {of this interdependence} . . . because of the secret ways in which Middle Eastern crude is traded," he said.

As tensions with Iran have risen, the Reagan administration has considered an embargo against Iranian goods, particularly oil. The review gained momentum in August, after France banned purchases of Iranian oil, industry sources said. There have been no import restrictions since the agreement accompanying the release of U.S. hostages from Tehran in 1981, Roberts noted.

Last week, an aide to Senate Minority Leader Robert J. Dole (R-Kan.) said it is "fairly certain" that Dole will introduce legislation this week to embargo all U.S. trade with Iran, according to a wire service report.

A U.S. embargo on Iranian oil imports would be little more than a symbolic gesture, as was an earlier U.S. decision to ban imports of Libyan oil, according to industry experts.

"There would be no effect on the oil market, because the Iranian oil would find a home somewhere else," said Philip K. Verleger, a visiting fellow with the Institute for International Economics in Washington.

But a U.S. oil embargo on Iranian crude would likely force Iran to cut its oil prices to other buyers, perhaps kicking off another round of price-cutting by members of the Organization of Petroleum Exporting Countries, Verleger said. "That would be good for consumers and bad for U.S. {oil} producers."

Iran's desperate need for revenue to finance its war with Iraq is at the core of its sharp increase in exports, Roberts noted. Although OPEC members had agreed to limit oil production by following nation-by-nation quotas, Iran and several other Persian Gulf nations broke the agreement and boosted their output this summer, Verleger said.

At the same time, oil buyers around the world went on a buying spree, said Verleger and Roberts, prompted by growing fears of warfare in the Persian Gulf following the Iraqi attack on the USS Stark in May and the Iranian mining of tanker shipping lanes in the area.

"The oil traders sought to gain from the price rise sparked by the attack on the Stark," said Joseph C. Story, president of Gulf Consulting Services in McLean, who was quoted in Roberts' analysis. "The Japanese pushed the price up with panic buying and traders bought a lot of premium oil from the Mideast, including Iran," Story said.

Verleger said Iran may lose its position as a prime U.S. supplier even if there is no embargo. There are indications that the rush by oil importers to fill inventories for the fall is tapering off now, he said, and the authoritative Petroleum Intelligence Weekly newsletter estimates Iran's overall sales are down to 1.5 million barrels a day, from 2 million barrels daily in August.

Last year, Iran's exports to the United States averaged only one-seventh what they did in July, or 90,000 barrels a day, a level that placed it far behind Canada, Mexico, Nigeria and Saudi Arabia, the customary prime suppliers.