WASHINGTON — Russia’s second-largest oil company has suspended development of an oil field in western Iran, marking the first time a Russian concern is applying economic pressure on the Islamic Republic.

Citing American sanctions on foreign oil companies that invest more than $20 million in the Iranian economy, the vice president of Lukoil, Leonid Fedun, announced yesterday that work on the Anaran field has stopped.

“We opened the largest field in Iran, but we can’t work there because the U.S. State Department has banned third countries from investing more than $20 million,” Mr. Fedun said at a press conference in Moscow.

The announcement comes on the heels of President Putin’s icy visit last week to Tehran, at which he reportedly offered the Iranians a deal, in exchange for Iran suspending its enrichment of uranium, and was spurned.

Over the weekend, Ali Larijani resigned as Iran’s chief nuclear negotiator, though he remains Iran’s equivalent to a national security adviser. In Mr. Larijani’s place, President Ahmadinejad appointed the deputy foreign minister, Saeed Jalili, whom Western analysts consider a hard-liner.

If other Russian companies, such as Gazprom, follow Lukoil’s lead, Iran will soon be left with only China’s diplomatic support within the U.N. Security Council. To date, the Russians have opposed serious U.N. sanctions against Tehran and have sold Iran and Iran’s ally Syria advanced air defense systems.

Yesterday, the top foreign policy adviser to Iran’s supreme leader, Ayatollah Ali Khamenei, criticized Mr. Ahmadinejad’s appointment of Mr. Jalili. “It seems that if this had not happened, it would have been better,” Ali Akbar Velayati said, Agence France-Presse reported. “In the very important and sensitive situation where the nuclear issue is at the moment, it would be better if this did not happen, or at least it was prevented,” he added, referring to Mr. Larijani’s resignation.

Mr. Larijani may have resigned because the Russian offer was rejected, the deputy director of the Washington Institute for Near East Policy, Patrick Clawson, said. “The assumption is that the Larijani departure was because Larijani was angry that Putin’s offer was not getting serious attention,” Mr. Clawson said.

Mr. Fedun made reference to the American threat to bar foreign companies from its financial markets as the reason for Lukoil’s decision to suspend work in Anaran. Lukoil controlled 25% of the project; the other 75% was controlled by Norsk Hydro.

Lukoil and Houston-based ConocoPhillips merged in 2004, with the latter owning about 20% of Lukoil’s stock. Lukoil now operates about 2,000 retail gas stations in America.

Lukoil and other Russian energy companies still have ongoing business developing Iranian oil and natural gas sectors. But the public announcement of the Anaran suspension is important, Mr. Clawson said.

“Even if Lukoil is using this as an excuse, that’s great news,” he said, “because that will say to the Iranian officials, ‘You are paying a high price for your nuclear policies.'”

Mr. Clawson added: “This is the first time the Russians have divested and the first time they are announcing this publicly. I think at the end of the day, it will be really hard for Ahmadinejad’s line to prevail.”

A State Department official said it was seeking more information on the Lukoil announcement. “We need more clarity to assess this development,” the official said. “The fact that the company is raising this issue publicly may have a ripple effect, but it’s still too early to say.”