The Clinton administration denied permission for an Iranian diplomat to address an oil industry conference in Washington last week despite a new policy of encouraging increased contacts with Iran.

A State Department official confirmed that Iran's U.N. ambassador, Mohammad Hadi Nejadhosseinian, was refused permission to travel to the conference from the mission's offices in New York, citing concerns that his participation would not be in the ''U.S. national interest.''Mr. Nejadhosseinian had been invited by Petroleum Finance Co. to speak as part of a panel on Caspian Sea oil development. But authorization was denied ''because Iranian participation in the Caspian Sea oil area from our perspective is not a good thing,'' the State Department official said.

Organizers subsequently offered the diplomat a chance to address the conference by a televised hookup. Mr. Nejadhosseinian declined because the arrangement would have been ''in defiance'' of the State Department's decision, said Vahan Zanoyan, president of Petroleum Finance, a Washington-based consulting firm.

An official at the State Department called the ambassador's action a ''useful thing'' and ''an interesting statement.'' But U.S. oil industry officials and analysts blasted the administration's behavior.

''The Iranians have shown both tact and maturity. It's a shame that the White House didn't do the same,'' said J. Robinson West, Petroleum Finance's chairman, and a former assistant secretary of the interior. A week earlier, Mr. Nejadhosseinian had received permission to attend a meeting in Los Angeles on political developments since the recent warming in relations with the United States. Iranian diplomats are generally free to travel only within a 25-mile radius of New York.

The State Department deflected questions about the consistency of the new policy of allowing more visas and contacts with Iranians, saying that requests are being considered on a case-by-case basis. Few were buying the explanation.

''It's madness,'' said Geoffrey Kemp, a former National Security Council official, now director of regional strategic programs at the Nixon Center for Peace and Freedom, who recently attended a conference in Iran.

One factor that may have figured into the State Department's decision is Iran's opposition to a trans-Caspian pipeline that would link Central Asia with the Caucasus, a primary U.S. policy goal. The proposed line forms a vital extension of an east-west export route through Turkey, which would avoid north-south lines through either Russia or Iran.

Iranian Foreign Minister Kamal Kharrazi issued a statement against the trans-Caspian pipeline Tuesday, saying that it would violate the rights of shoreline states, which have yet to agree on a legal division of the waterway. A spokesman for Iran's U.N. mission could not be reached for comment.

But the administration's decision to bar Mr. Nejadhosseinian's appearance was seen as another example of administration flip-flops on its friendlier Iran policy.

Recently officials rushed to counter bad publicity after an Iranian wrestling team invited to the United States as a good-will gesture was detained and fingerprinted by the Immigration and Naturalization Service.

The administration also struggled this month to deal with several U-turns on the creation of a Radio Free Iran service, which congressional backers want to broadcast anti-government propaganda to Iranian audiences. Mideast experts see the plan as undercutting efforts to improve relations.