Iran's Ambassador to Russia Seyed Mahmoud-Reza Sajjadi (R) at a news conference in Moscow on Feb. 16. Sajjadi said plans to cut off supplies of Iranian crude to Europe would only benefit the Islamic republic, which in the past has been heavily dependent on imported fuel due to restricted refining capacity. (REUTERS)

International sanctions on Iran are starting to pinch, and Tehran is scrambling to hang on to buyers for its crude oil exports.

In January, China, South Korea and Singapore sharply cut their oil purchases from Iran. Last month, Shipping Corp. of India canceled an Iranian shipment because its European insurers refused to provide coverage for the tanker, according to Lloyd’s List. And Japanese oil refiners have asked for clauses to be added to oil-purchase contracts so they can back out if they can’t obtain tanker insurance.

“Iran is scrambling to find buyers, but other countries are also scrambling to diversify away from what they see as risky supply,” said Richard Meade, editor of Lloyd’s List.

In addition, the Angolan state-owned oil company Sonangol recently dropped out of an Iranian project to expand natural gas production in the South Pars field. Sonangol said that sanctions would make it difficult to finance its 20 percent stake in the $7.5 billion project.

For the moment, however, Iran is profiting from rising tensions, which have driven oil prices up. Moreover, it isn’t clear whether the volume of Iran’s oil exports has been reduced yet. Although exports through mid-February were lower than last year’s average, fluctuations aren’t unusual.

But traders and oil experts say that Iran will find it increasingly difficult to keep up its exports as the July 1 European Union oil embargo approaches and as the United States tightens restrictions on bank transactions with Iran. There are 13 international insurance groups — “protection and indemnity clubs” — that provide insurance for 90 percent of global shipping, and underwriters will be limited by sanctions.

With U.S. crude oil prices around $107 a barrel, Obama administration officials are watching closely, worried that oil-market pressures are racing ahead of sanctions plans. U.S. and European officials had been hoping to wean Tehran’s European customers off Iranian petroleum between now and the E.U. embargo date, offsetting the loss of Iranian crude supplies with increases in output by Saudi Arabia and other oil exporters. Instead, U.S. officials now fear that the prospect of a supply shortfall, tight oil markets and price increases in an election year.

“The sanctions [on the Central Bank of Iran] don’t kick in until June 28, but the insurers are already making their moves,” said a senior administration official, speaking on the condition of anonymity because he was not authorized to talk publicly about the events. “Already we’re seeing full tankers being parked because they can’t unload.”

PFC Energy, a Washington consulting firm, says that Iran has 35 million barrels in floating storage, about half its capacity. Oil traders at one European firm said four tankers are searching for a place to unload Iranian crude.

To make matters worse, the sidelining of Iranian oil is occurring when oil supplies are tight, the official said. Oil shipments from South Sudan, Yemen and Syria have all but stopped because of civil strife or sanctions; together they accounted for more than half a million barrels a day of oil exports, almost as much as Iran’s exports to Europe.

“There’s a perfect storm brewing,” the official said.

Concern about spiraling oil ­prices lay behind the administration’s initial hesitancy to embrace sanctions against the Central Bank of Iran, which facilitates Iran’s foreign oil sales. Treasury Secretary Timothy F. Geithner, in a Dec. 1 letter to Congress, said that the administration needed flexibility in how it imposed sanctions that ­impede the flow of Iranian oil, because of uncertainty about how the markets would respond.

Geithner warned that proposed legislation mandating sanctions against the Central Bank of Iran “could have the opposite effect from what is intended, and increase the Iranian regime’s revenue.” The bill sailed through Congress days later with overwhelming bipartisan support.

A second U.S. official expressed chagrin over what he called Iran’s expert manipulation of the oil markets, using well-timed threats to prop up oil prices.

“Iran’s threats have driven up prices, and it’s clearly deliberate,” the official said. He dismissed Iran’s threat in December to shut down the Strait of Hormuz as a tactic employed by Iranian officials to boost oil income. “They were never going to close the strait,” he said.

Iran has been deploying all its resources to keep exports flowing. The state-owned shipping company NITC owns more than 50 tankers, which Iran insures ­itself. Iran continues to load oil into the Sumed pipeline, mixing it with Saudi petroleum going from the Gulf of Suez to the Mediterranean.

While China’s imports of Iranian crude fell to 252,000 barrels a day in January, less than half of last year’s average, India’s imports from Iran surged to a level two-thirds higher than last year’s average, according to Lloyd’s List.

But India’s imports fell early last month and other buyers of Iranian crude oil have reduced contract volumes starting in April, according to European oil traders who asked to remain anonymous to protect their business relationships.

Roger Diwan, an oil expert at PFC Energy, said Iran would have trouble finding customers with an appetite for the more than 600,000 barrels a day that Europe will no longer import. Big importers such as China and Japan are unlikely to give up secure sources like Saudi Arabia or to fill strategic reserves at high prices. China is pressing Iran for lower prices, but Iran does not want to discount its oil.

“Who else can buy their barrels? Zimbabwe? North Korea?” Diwan said. “Will they smuggle more to Pakistan and Turkey? Yes, but that’s small stuff.”

He said that before the end of the year, “the Iranians will have half a million barrels a day shut in.”