Energy Secretary James R. Schlesinger, said yesterday that the current Iranian oil crisis is "more serious" than the 1973 Arab embargo.

"It would be unwise to assume that Iran will come [back] on line soon," Schlesinger told the Senate Energy Committee, adding that mandatory conservation measures such as closing gasoline stations on Sundays could be necessary by summer.

To date, this is the most pessimistic assessment offered by the administration of the Iranian oil shutdown, which has pushed the world toward a severe shortage.

While Schlesinger recited previous plans to call on the Congress to approve mandatory conservation measures, such as the Sunday gasoline ban, he also testifed that the administration does not plan to call for gasoline coupon rationing.

Schlesinger was also the first administration official to confirm on the record that Saudi Arabia has placed a ceiling of 9.5 million barrels a day on its oil prodction. Until last month, Saudi production had gone up to 10.5 billion barrels a day.

Earlier this week, David J. Bardin, a senior Department of Energy official, told the commiteee that there was no indication of a Saudi cutback.

Sen. Mark O. Hatfield (R-Ore.), ranking minority member of the committee, was critical of DOE's planning efforts. "We can see no systematic, step-by-step plan for analysis and action in a highly probable energy emergency," he said.

Schlesinger told the committee that the administration hoped voluntary measures would make up for the 500,000 to one million barrels a day shortfall caused by the Iranian shutdown. He also said that shifts to domestic natural gas and coal would reduce oil demand.

"We do not know the extent to which we would need to turn to mandatory measures if, indeed, they are required," Schlesinger said.

Asked about U.S. participation in the International Energy Program (IEP), run by the 13-member International Energy Agency in Paris, Schlesinger said the sharing plan "would not be triggered until later in the year," if at all. Under the IEP, the Unitd States would be called upon to initially reduce demand by 500,000 barrels a day if sharing is called for.

Schlesinger told the committee that the International Energy Agency will meet March 1 to assess the oil situation in the industrial countries. He said that under the terms of the existing agreement, the United States would probably be a "net contributor" and would be forced to reduce consumption.

In recent days the White House has called for strict energy conservation measures at federal facilites, and has made a private appeal to industry to voluntarily cut back energy use to prevent mandatory measures.

In an exchange with Sen. Lowell P. Weicker (R-Conn.), Schlesinger said he "personally" favored some form of mandatory conservation measures now, but was not speaking for the administration.