The Reagan administration, stepping up pressure on Iran, said yesterday that unless Tehran responds positively by Friday to a United Nations demand for a cease-fire in the Persian Gulf war it will move formally next week to have the world body impose a mandatory arms embargo on Iran.

State Department spokeswoman Phyllis Oakley said Iran must give a "definitive" response to the Security Council's July 20 resolution calling for a cease-fire by "early next week."

"If they continue to give no definitive response to the resolution, then we believe formal drafting of a second resolution, calling for enforcement measures against any party which has not accepted the resolution should begin immediately thereafter," she said.

At the same time, a senior State Department official said the United States favors imposing an embargo on arms for Iran, rather than on its oil exports, because the latter measure would be too difficult to monitor.

"The sanction we think is the most practical is that of an arms embargo because control of the international oil market and direction of that market is . . . almost unimaginably difficult," said Assistant Secretary of State Richard W. Murphy.

His remarks seemed to indicate the State Department at least has rejected a proposal, under discussion within the Reagan administration the past several months, that the United States stop importing Iranian oil, which accounted for about three-quarters of the $612 million total U.S. imports from Iran last year.

The State Department official said the United States had taken "a hard look" at the embargo imposed by the Reagan administration on Libyan oil over a year ago and concluded it was "extraordinarily difficult" to make such a measure work because the oil was "fungible."

Therefore, it could simply be sold to other countries or on the international "spot" market for crude oil, he said.

However, an aide said later that Murphy's views on the impracticality of a U.S. oil embargo on Iran did not reflect a final administration decision on the proposal.

Murphy was interviewed by Arab reporters on the U.S. Information Agency's Worldnet system of satellite-transmitted television relayed to Arab gulf states.

Iraq has already said it will accept a comprehensive cease-fire in the seven-year-long war if Iran does, but not a partial one that applies only to the gulf and excludes the land war.

On Saturday, the Iraqis resumed their attacks on Iranian oil facilities and tankers in the gulf after a 45-day cease-fire in the so-called "tanker war."

Iraq has rejected a U.S. request to end the attacks that Oakley and other administration spokesmen admit have added a new urgency to diplomatic efforts to halt the gulf conflict.

On Monday, Iran's parliament speaker Hojatoleslam Ali Akbar Hashemi Rafsanjani said Tehran is ready for "all-out cooperation with the Security Council" but only if it condemned Iraq for starting the war.

Oakley said the United States realized that any agreement on an arms embargo among the U.N. Security Council's five permanent members, including the Soviet Union and China, would take "some time" and involve "a complicated consultative process."

Murphy, without mentioning the Soviets, who have already said they believe discussion of an arms embargo is "premature," told his Arab audience on Worldnet that "unfortunately" some council members were "grasping at straws" to avoid discussion of it.