The National Iranian Oil Co. notified American oil companies yesterday they could no longer buy Iranian oil even if it is destined for countries other than the United States.

This additional disruption of world oil supply channels, which the companies had feared after President Carter embargoed Iranian oil shipments to the U.S. earlier this week, will mean higher oil prices with more being sold to spot market prices of$40 to $45 a barrel.

White House press secretary Jody Powell said some U.S. allies have said they, too, will limit their imports of Iranian oil. Powell declined to name the other nations and did not indicate whether they planned to cut imports from Iran or end them altogether.

Meanwhile, Special Correspondent Nicholas Cumming-Bruce reported yesterday from Tehran that Iran's Revolutionary Council and the state oil company are considering longer-term cutbacks in oil exports.

Whatever their decision, Revolutionary Council members Sadegh Ghotbzadeh said, NIOC would lower production during the next "15 days to three weeks" while new customers are found for the 700,000 barrels a day previously headed for the U.S.

Carter administration officials are worried about a possible interruption in Iranian production, either as a matter of deliberate policy or as a result of unrest among oil field workers.

"There is very little spare (production) capacity to make up for Iran if they stop producing," a senior State Department official warned. He estimated other OPEC countries could increase their output by no more than 3.2 million barrels a day, and not even that much on a sustained basis. Iran has been producing between 3 million and 4 million barrels a day.

Because of the danger of an Iranian shutdown, the administration is making contingency plans to deal with any serious supply interruption. A wide range of proposals -- everything from new gasoline taxes to putting nuclear power plants now closed back on line -- is included in a paper drafted by Deputy Energy Secretary John Sawhill.

"This is the early stages of a pretty broad exercise," cautioned one administration energy expert. "It makes sense to take a look at everything, but I don't think anything has gotten to the point of a decision."

Cutting off U.S. oil companies from Iranian oil will have no serious effects on world oil supplies unless Iranian production also falls, one oil company executive said. "If Iran does not shut in, and people don't panic, I don't think there will be any shortage," declared J. V. D. Fear, marketing vice president of Sun. Co.

Exxon, Charter Co. and Ashland Oil were among the companies notified yesterday no more Iranian oil was available to them. "This is to inform you that in view of recent events we are not in a position to continue delivery of oil to you. Therefore your nominations (contracts) are deemed to be cancelled," read the messages sent by NIOC.

Iranian Oil Minister Ali Akbar Moinfar claims Iran has reached agreements to sell 25 percent of the crude formerly taken by the U.S., "a large part of it on the spot market at $45 per barrel," it was reported yesterday.

In the same press conference, however, Moinfar also said there are no plans to lower oil production "because we do not want the slightest damage to be done to other countries." This assertion conflicted with other reports from Teheran, reflecting the chaotic state of policymaking there.

The most likely buyers of the oil are Japanese companies who have been asked by Iran to increase their purchases, which most observers thought they would do, even at$45 a barrel.

Japan, which has virtually no oil production, is trying to increase it oil stocks as rapidly as possible spurred by the same fear of an Iranian shutdown. s

Whether Iran slashes its exports does not depend entirely on orders from Ayatollah Khomeini, or the Islamic government's Revolutionary Council, analysts said.

Activists in the oil-rich Khuzestan province have repeatedly mounted strikes in the oil fields against Tehran's labor policies, pay, and working conditions. Khuzestan one of Iran's major Arab-speaking provinces, has also been the target of activity by the Marxist Tudeh party, which earlier this year claimed credit for disrupting oil field operations.

Iranian officials continue to insist there are no major technical problems in maintaining Iran's current 3.7 million barrel a day production level, despite frequent reports to the contrary.

Meanwhile, in a related development, the Senate Energy Committee has called closed hearings to receive administration testimony on an amendment offered by Sen. Bob Dole (R-Kansas) and Sen. Bill Bradley (D-N.J.) requiring the Energy Department to buy oil for the U.S. strategic petroleum reserve. The Carter administration has stopped further purchases for their reserve partly because of pressure from Saudi Arabia.